Both Water and Land: Animals
Chapter
19
Outline 
	19.1	Evolution of Animals
	•	Representatives of today’s animal phyla suddenly appear in the Cambrian period.307 
	•	Seven key innovations apparently occurred during the evolution of animals.308
	19.2	Introducing the Invertebrates
	•	Sponges are multicellular.310
	•	Cnidarians have true tissues.310–11
	•	Flatworms have bilateral symmetry.311
	•	Roundworms have a pseudocoelom.312
	19.3	Protostomes and Deuterostomes Compared
	•	Developmental differences distinguish protostomes from deuterostomes.313
	•	Both protostomes and deuterostomes have a coelom.313
	19.4	Molluscs, Annelids, and Arthropods
	•	Molluscs, annelids (segmented), and arthropods (jointed appendages) are protostomes.314 
	19.5	Echinoderms and Chordates
	•	Echinoderms and chordates are deuterostomes.319 
	•	All the vertebrates (fishes, amphibians, reptiles, and mammals) are chordates.321
	19.6	Human Evolution
	•	Australopithecines, Homo habilis, Homo erectus, and the Neandertals preceded the evolution of Homo sapiens.329–32
Classification isn’t all that easy. A duckbill platypus has webbed feet and a paddlelike tail for swimming. It uses its bill to find and collect food. Millions of tiny, jelly-filled pits lining the toothless bill are electroreceptors that detect tiny electrical charges given off by the platypus’s prey. It eats worms, crustaceans, fish, tadpoles, and adult frogs, which it grinds with two hornlike plates at the back of its bill. To reproduce, the duckbill platypus lays hard-shelled eggs. 
Somewhat surprisingly, the duckbill platypus is classified as a mammal, the same as a cat. The two characteristics of a mammal are hair and mammary glands, and the duckbill platypus has both. Thick hair covers its body, except for its bill and feet. Female platypuses produce milk from mammary glands, but because they have no nipples, the young lick the milk from their fur.
It’s clear to us that gulls are birds. After all, they fly in the air. But penguins don’t fly. When on land, they walk around on webbed hind feet with a flipper hanging at each side. Penguins are excellent divers and spend most of their time at sea fishing for fish, squid, and small crustaceans called krill. The Emperor penguin has been known to dive to a depth of more than 1,500 feet and stay down for over 18 minutes. Penguins’ solid bones—not hollow like those of other birds—help them stay underwater.
So what makes a penguin a bird? Like the gull, the attractive coat of a penguin is made of feathers, and its flipper, built like a bird’s wing, allows it to “fly” through the water. Penguins lay hard-shelled eggs and raise their chicks on land. A penguin might look like a mammal from a distance, but it is clearly a bird! 
The content of this chapter will help you learn to classify some of the other major groups of animals. 
19.1		Evolution of Animals 
Animals can be contrasted with plants and fungi. Like both of these, animals are multicellular eukaryotes, but unlike plants, which make their food through photosynthesis, they are chemoheterotrophs and must acquire nutrients from an external source. So must fungi, but fungi digest their food externally and absorb the breakdown products. Animals ingest (eat) their food and digest it internally. 
Animals usually carry on sexual reproduction and begin life only as a fertilized diploid egg. From this starting point, they undergo a series of developmental stages to produce an organism that has specialized tissues, usually within organs that carry on specific functions. Two types of tissues in particular—muscles and nerves—characterize animals. The presence of these tissues allows an animal to exhibit motility and a variety of flexible movements. The evolution of these tissues enables many types of animals to search actively for their food and to prey on other organisms. Coordinated movements also allow animals to seek mates, shelter, and a suitable climate—behaviors that have resulted in the vast diversity of animals. The more than 30 animal phyla we recognize today are believed to have evolved from a single ancestor.
Figure 19.1 illustrates the characteristics of a complex animal using the frog as an example. A frog goes through a number of embryonic stages to become a larval form (the tadpole) with specialized organs, including muscular and nervous systems that enable it to swim. A larva is an immature stage that typically lives in a different habitat and feeds on different foods than the adult. By means of a change in body form called metamorphosis, the larva, which only swims, turns into a sexually mature adult frog that swims and hops. The aquatic tadpole lives on plankton, and the terrestrial adult typically feeds on insects and worms. A large African bullfrog will try to eat just about anything, including other frogs, as well as small fish, reptiles, and mammals.
The Evolutionary History of Animals
Animals are believed to have evolved from a protistan ancestor over 600 mya, but the exact time of their appearance is not known. Figure 19.2 shows that the seas teemed with strange-looking invertebrates (animals without an endoskeleton [internal skeleton] of bone or cartilage) during the Cambrian period. Animal life was sparse before this period, and then suddenly representatives of all of today’s animal phyla appear in the fossil record within some 10 million years. The appearance of this great variety of animals is so dramatic that the event is called the Cambrian explosion. 
Paleontologists ask, “Why are animal fossils suddenly prevalent during this period?” It could be that only during the Cambrian period did animals evolve a protective exoskeleton (external skeleton), hard portions able to survive the forces that are apt to destroy fossils. Perhaps oxygen gas, pumped into the atmosphere by cyanobacteria and algae, was in sufficient quantity to permit aquatic animals to acquire oxygen, even though they were covered by hard outer skeletons. Then, too, predation may have played a role: Skeletons may have evolved during the Cambrian period because skeletons help protect animals from other animals.
The Evolutionary Tree 
of Animals
Without an adequate fossil record, it has been impossible to trace the evolutionary history of animals with certainty. However, the developmental stages of today’s living animals point to seven evolutionary trends that allowed animals to become more complex (Fig. 19.3). 
Evolutionary Trends
Animals differ in biological organization. Sponges, like all animals, are  multicellular, but they have no true tissues and therefore have the cellular level of organization. Cnidarians, such as Hydra, with two tissue layers in their body wall, have  true tissues, which can be associated with two germ layers when they are embryos. The animals in all the other phyla have three germ layers called ectoderm, endoderm, mesoderm as embryos, and these shape their organs as they develop.
Animals differ in symmetry. Sponges are examples of animals that have no particular symmetry, and are therefore asymmetrical. Radial symmetry, as seen in Hydra, means that the animal is organized circularly, similar to a wheel. No matter where the animal is sliced longitudinally, two mirror images are obtained (Fig. 19.4a). Bilateral symmetry, as seen in flatworms, means that the animal has definite right and left halves; only a longitudinal cut down the center of the animal will produce mirror images (Fig. 19.4b). During the evolution of animals, the trend toward  bilateral symmetry is accompanied by cephalization, localization of a brain and specialized sensory organs at the anterior end of an animal.
Animals differ with regard to   a body cavity. Some animals have no body cavity and are acoelomates as are flatworms (Fig. 19.5a). Acoelomates are packed solid with mesoderm. In contrast, a body cavity provides a space for the various internal organs.  Roundworms are pseudocoelomates, and their body cavity is incompletely lined by mesoderm—that is, a layer of mesoderm exists beneath the body wall but not around the gut (Fig. 19.5b). Most animals are  coelomates in which the body cavity is completely lined with mesoderm (Fig. 19.5c). In animals with a coelom, mesentery, which is composed of strings of mesoderm, supports the internal organs. In coelomate animals, such as earthworms, lobsters, and humans, the mesoderm can interact not only with the ectoderm but also with the endoderm. Therefore, body movements are freer because the outer wall can move independently of the organs, and the organs have the space to become more complex. In animals without a skeleton, a coelom even acts as a so-called hydrostatic skeleton.
Animals can be nonsegmented or  segmented. Segmentation is the repetition of body parts along the length of the body. Annelids, such as an earthworm; arthropods, such as a lobster; and chordates, such as ourselves, are segmented. To illustrate your segmentation, run your hand along your backbone, which is composed of a series of vertebrae. Segmentation leads to specialization of parts because the various segments can become differentiated for specific purposes.
Two groups of animals, the arthropods and chordates, have  jointed appendages, which are particularly useful for movement on land. A lobster has a jointed exoskeleton, while a human has a jointed endoskeleton.
Molecular Data
Modern phylogenetic investigations take into account molecular data, primarily nucleotide sequences, when classifying animals. It is assumed that the more closely related two organisms are, the more nucleotide sequences they will have in common. An evolutionary tree based on molecular data is quite different from the one based only on anatomical characteristics that we have been discussing.
In the traditional tree, the protostomes are restricted to three phyla which have a true coelom (see Fig. 19.3). So, for example, flatworms, which are acoelomate, are not protostomes, whereas segmented worms are protostomes because they have a coelom, and their development follows a particular pattern.
Figure 19.6 shows an evolutionary tree based on molecular data. These data suggest that many more animal phyla should be designated protostomes because their rRNA sequences are so similar. However, the protostomes are divided into two groups. One group has a particular type of immature stage, the trochophore larva, as reflected in their name, lophotrochozoa. (The lopho stands for a feeding apparatus called a lophophore, which is seen in some animal phyla we are not studying.) The other group contains the roundworms and the arthropods. Both of these types of animals shed their outer covering as they grow; therefore, they are called ecdysozoa, which means molting animals.
Notice, too, that segmentation doesn’t play a defining role in the phylogenetic tree based on molecular data. In the traditional tree, the segmented worms (e.g., earthworm) are placed close to the arthropods, which are also segmented. In the new tree, the segmented worms are lophotrochozoa, and the arthropods are ecdysozoa.
19.2		Introducing the Invertebrates 
This section describes the invertebrate animals—the sponges, cnidarians, flatworms, and roundworms. Several key evolutionary trends are seen among these phyla (see Fig. 19.3). 
Sponges: Multicellularity
Sponges are placed in phylum Porifera because their saclike bodies are perforated by many pores (Fig. 19.7a). Sponges are aquatic, largely marine animals that vary greatly in size, shape, and color. Sponges are multicellular but lack organized tissues. Therefore, sponges have the cellular level of organization. Evolutionists believe that sponges are outside the mainstream of the rest of animal evolution. 
The interior of a sponge is lined with flagellated cells called collar cells, or choanocytes. The -beating of the flagella produces water currents that flow through the pores into the central cavity and out through the osculum, the upper opening of the body. Even a simple sponge only 10 centimeters tall is estimated to filter as much as 100 liters of water each day. It takes this much water to supply the needs of the sponge. A sponge is a sedentary filter feeder, an organism that filters its food from the water by means of a straining device—in this case, the pores of the walls and the microvilli making up the collar of collar cells (Fig. 19.7b). Microscopic food particles that pass between the microvilli are engulfed by the collar cells and digested by them in food vacuoles.
Sponges can reproduce both asexually and sexually. They reproduce asexually by fragmentation or by budding. During budding, a small protuberance appears and gradually increases in size until a complete organism forms. Budding produces colonies of sponges that can become quite large. During sexual reproduction, eggs and sperm are released into the central cavity, and the zygote develops into a flagellated larva that may swim to a new -location. If the cells of a sponge are mechanically separated, they will reassemble into a complete and functioning organism! Like many less specialized organisms, sponges are also capable of regeneration, or growth of a whole from a small part.
Some sponges have an endoskeleton composed of spicules, small needle-shaped structures with one to six rays. Most sponges have fibers of spongin, a modified form of collagen; a bath sponge is the dried spongin skeleton from which all living tissue has been removed. Today, however, commercial “sponges” are usually synthetic.
Cnidarians: True Tissues
Cnidarians (phylum Cnidaria) are an ancient group of invertebrates with a rich fossil record. Cnid-arians are radially sym-met-rical and capture their prey with a ring of tentacles that bear specialized stinging cells, called cnidocytes (Fig. 19.8). Each cnidocyte has a capsule called a nematocyst, containing a long, spirally coiled, hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some nematocysts merely trap a prey or predator; others have spines that penetrate and inject paralyzing toxins before the prey is captured and drawn into the gastrovascular cavity. Most cnidarians live in the sea, though there are a few freshwater species. 
During devel---opment, cnidarians have two germ layers -(ecto-derm and endoderm), and as adults cnidarians have the tis--sue level of organization. Two basic body forms are seen among cnidarians—the polyp and the medusa. The mouth of a polyp is directed upward from the substrate, while the mouth of the medusa is directed downward. A medusa has much jellylike packing material and is commonly called a “jellyfish.” Polyps are tubular and generally attached to a rock (Fig. 19.9).
Cnidarians, as well as other marine animals, have been the source of medicines, particularly drugs that counter inflamma-tion.
Flatworms: Bilateral Symmetry
Flatworms (phylum Platyhel-minthes) have bilateral symmetry. Like all the other animal phyla we will study, they also have three germ layers. However, the flatworms have no coelom—they are acoelomates. The presence of mesoderm in addition to ectoderm and endoderm gives bulk to the animal and leads to greater complexity. 
Free-living flatworms, called planarians, have several body systems (Fig. 19.10), including a ladderlike nervous system. A small anterior brain and two lateral nerve cords are joined by cross-branches called transverse nerves. Planarians exhibit cephalization; aside from a brain, the “head” end has light-sensitive organs (the eyespots) and chemosensitive organs located on the auricles. Their three muscle layers—an outer circular layer, an inner longitudinal layer, and a diagonal layer—allow for varied movement. Their ciliated epidermis allows planarians to glide along a film of mucus. 
The animal captures food by wrapping itself around the prey, entangling it in slime, and pinning it down. Then the planarian extends a muscular pharynx, and by a sucking motion tears up and swallows its food. The pharynx leads into a three-branched gastrovascular cavity where digestion occurs. The digestive tract is incomplete because it has only one opening.
Planarians are hermaphrodites, meaning that they possess both male and female sex organs. The worms practice cross--fertil-ization: The penis of one is inserted into the genital pore of the other, and a reciprocal transfer of sperm takes place. The fertilized eggs hatch in 2–3 weeks as tiny worms.
The parasitic flatworms belong to two classes: the tapeworms and the flukes. As adults, tapeworms are endoparasites (internal parasites) of various vertebrates, including humans (Fig. 19.11a). They vary in size from a few millimeters to nearly 20 meters. Tapeworms have a well-developed anterior region called the scolex, which bears hooks and suckers for attachment to the intestinal wall of the host. Behind the scolex, a series of reproductive units called proglottids contain a full set of female and male sex organs. After fertilization, the organs within a proglottid disintegrate, and it becomes filled with mature eggs. The eggs, or else the mature proglottids, are eliminated in the feces of the host.
Flukes are all endoparasites of various vertebrates. The anterior end of these animals has an oral sucker and at least one other sucker used for attachment to the host. Flukes are usually named for the organ they inhabit; for example, there are blood, liver, and lung flukes. The blood fluke (Schistosoma spp-.) occurs predominantly in the Middle East, Asia, and Africa. Nearly 800,000 persons die each year from infection by this fluke, called schistosomiasis. Adults are small (approximately 2.5 centimeters long) and may live for years in their human hosts (Fig. 19.11b). 
Roundworms: Pseudocoelomates 
Roundworms (phylum Nematoda) possess two anatomical features not seen in animals discussed previously: a body cavity and a complete digestive tract (Fig. 19.12). The body cavity is a pseudocoelom and is incompletely lined with mesoderm (see Fig. 19.5).  The digestive tract is complete because it has both a mouth and an anus. Worms in general do not have a skeleton, but the fluid-filled pseudocoelom supports muscle contraction and enhances flexibility.
The roundworms are nonsegmented, meaning that they have a smooth outside body wall. Roundworms are generally colorless and less than 5 centimeters in length, and they occur almost everywhere—in the sea, in fresh water, and in the soil—in such numbers that thousands of them can be found in a small area. Many are free-living and feed on algae, fungi, microscopic animals, dead organisms, and plant juices, causing great agricultural damage. Parasitic roundworms live anaerobically in every type of animal and many plants. Several parasitic roundworms infect humans. 
A female Ascaris, a human parasite, is very prolific, producing over 200,000 eggs daily. The eggs are eliminated with host feces, and under the right conditions they can develop into a worm within two weeks. The eggs enter the body via uncooked vegetables, soiled fingers, or ingested fecal material, and hatch in the intestines. The juveniles make their way into the cardiovascular system and are carried to the heart and lungs. From the lungs, the larvae travel up the trachea, where they are swallowed and eventually reach the intestines. There the larvae mature and begin feeding on intestinal contents. 
Trichinosis is a fairly serious human infection rarely seen in the United States. The female trichina worm burrows into the wall of the host’s small intestine; here she deposits live larvae that are carried by the bloodstream to the skeletal muscles, where they encyst. Once the adults are in the small intestine, digestive disorders, fatigue, and fever occur. After the larvae encyst, the symptoms include aching joints, muscle pain, and itchy skin. Humans catch the disease when they eat infected meat.
Elephantiasis is caused by a roundworm called a filarial worm, which utilizes mosquitoes as a secondary host. Because the adult worms reside in lymphatic vessels, fluid return is impeded, and the limbs of an infected human can swell to an enormous size, even resembling those of an elephant. When a mosquito bites an infected person, it can transport larvae to a new host.
Other roundworm infections are more common in the United States. Children frequently acquire pinworm infection, and hookworm is seen in the southern states, as well as worldwide. A hookworm infection can be very debilitating because the worms attach to the intestinal wall and feed on blood. Good hygiene, proper disposal of sewage, thorough cooking of meat, and regular deworming of pets usually protect people from parasitic roundworms.
19.3		Protostomes and Deuterostomes Compared
Protostomes and deuterostomes can be distinguished on the basis of embryological development. In this text, we will discuss molluscs, annelids, and arthropods as representative protostomes, and echinoderms and chordates as deuterostomes. The first embryonic opening in both protostomes and deuterostomes is called the blastopore. In protostomes (proto, before), the blastopore becomes the mouth, and in deuterostomes, it becomes the anus. The coelom develops differently in protostomes and deuterostomes (Fig. 19.13).
19.4		Molluscs, Annelids, and Arthropods
Traditionally, molluscs, annelids, and arthropods have been grouped together as protostomes.
Molluscs
Despite being a very large and diversified group, all molluscs (phylum Mollusca) have a body composed of at least three distinct parts: (1) The visceral mass is the soft-bodied portion that contains internal organs; (2) the foot is the strong, muscular portion used for locomotion; (3) the mantle is a membranous or sometimes muscular covering that envelops, but does not completely enclose, the visceral mass. In addition, the mantle cavity is the space between the two folds of the mantle. The mantle may secrete an exoskeleton called a shell. Another feature often present is a rasping, tonguelike radula, an organ that bears many rows of teeth and is used to obtain food (Fig. 19.14). 
In gastropods (meaning stomach-footed), including nudibranchs, conchs, and snails, the foot is ventrally flattened, and the animal moves by muscle contractions that pass along the foot (Fig. 19.15, top). Many gastropods are herbivores that use their radulas to scrape food from surfaces. Others are carnivores, using their radulas to bore through surfaces such as bivalve shells to obtain food. In snails that are terrestrial, the mantle is richly supplied with blood vessels and functions as a lung. 
In cephalopods (meaning head-footed), including octopuses, squids, and nautiluses, the foot has evolved into tentacles about the head (Fig. 19.15, middle). The tentacles seize prey, and then a powerful beak and a radula tear it apart. Cephalopods possess well-developed nervous systems and complex sensory organs. The large brain is formed from a fusion of ganglia, and nerves leaving the brain supply various parts of the body. An especially large pair of nerves controls the rapid contraction of the mantle, allowing these animals to move quickly by jet propulsion of water. Rapid movement and the secretion of a brown or black pigment from an ink gland help cephalopods escape their enemies. In the squid and octopus, a well-developed eye resembles that of vertebrates, having a cornea, lens, retina, and iris. Octopuses have no shell, and squids have only a remnant of one concealed beneath the skin. Scientific experiments reveal that octopuses in particular are highly intelligent.
Clams, oysters, scallops, and mussels are called bivalves because of the two parts to their shells (Fig. 19.15, bottom). They have a muscular foot that projects ventrally from the shell. In a clam, such as the fresh-water clam, the calcium carbonate shell has an inner layer of mother-of-pearl. The clam is a filter feeder. Food particles and water enter the mantle cavity by way of the incurrent siphon, a posterior opening between the two valves. Mucous secretions cause smaller particles to adhere to the gills, and ciliary action sweeps them toward the mouth.
Molluscs have some economic importance as a source of food and pearls. If a foreign body is placed between the mantle and the shell of a clam, concentric layers of shell are deposited about the particle to form a pearl. 
Annelids: Segmented Worms 
Annelids (phylum Annelida) are segmented, as can be seen externally by the rings that encircle the body of an earthworm. Partitions called septa divide the well-devel--oped, fluid-filled ---coe-lom, which is used as a hydrostatic skeleton to facilitate movement. In annelids, the complete digestive tract body plan has led to specialization of parts (Fig. 19.16). For example, the digestive system may include a pharynx, esophagus, crop, gizzard, intestine, and accessory glands. Annelids have an extensive closed circulatory system with blood vessels that run the length of the body and branch to every segment. The nervous system consists of a brain connected to a ventral nerve cord, with ganglia in each segment. The excretory system consists of nephridia in most segments. A nephridium is a tubule that collects waste material and excretes it through an opening in the body wall.
Most annelids are polychaetes (having many setae per segment) that live in marine environments. Setae are bristles that anchor the worm or help it move. The clam worm Nereis is a predator. It preys on crustaceans and other small animals, capturing them with a pair of strong, chitinous jaws that extend with a part of the pharynx (Fig. 19.17a). In support of its predatory way of life, Nereis has a well-defined head region, with eyes and other sense organs. Other polychaetes are sedentary (sessile) tube worms, with tentacles that form a funnel-shaped fan. Water currents created by the action of cilia trap food particles that are directed toward the mouth (Fig. 19.17b).
The oligochaetes (few setae per segment) include the earthworms. Earthworms do not have a well-developed head, and they reside in soil where there is adequate moisture to keep the body wall moist for gas exchange. They are scavengers that feed on leaves or any other organic matter, living or dead, that can conveniently be taken into the mouth along with dirt.
Leeches have no setae but have the same body plan as other annelids. Most are found in fresh water, but some are marine or even terrestrial. The medicinal leech can be as long as 20 centimeters, but most leeches are much shorter. A leech has two suckers, a small one around the mouth and a large posterior one. While some leeches are free-living, most are fluid feeders that penetrate the surface of an animal by using a proboscis or their jaws, and then suck in fluids with their powerful pharynx. Leeches are able to keep blood flowing and prevent clotting by means of a substance in their saliva known as hirudin, a powerful anticoagulant. This secretion has added to their potential usefulness in the field of medicine today (Fig. 19.17c). 
Arthropods: Jointed Appendages
Arthropods (phylum Arthro-poda) are extremely diverse. Over one million species have been discovered and described, but some experts suggest that as many as 30 million arthropods may exist—most of them insects. The success of arthropods can be attributed to the following six characteristics:
	1.	Jointed appendages. Basically hollow tubes moved by muscles, jointed appendages have become adapted to different means of locomotion, food gathering, and reproduction (Fig. 19.18). These modifications account for much of the diversity of arthropods.
	2.	Exoskeleton. A rigid but jointed exoskeleton is composed primarily of chitin, a strong, flexible, nitrogenous polysaccharide. The exoskeleton serves many functions, including protection, prevention of desiccation, attachment for muscles, and locomotion. 
			Because an exoskeleton is hard and nonexpandable, arthropods must undergo molting, or shedding of the exoskeleton, as they grow larger. During molting, arthropods are vulnerable and are attacked by many predators. 
	3.	Segmentation. In many species, the repeating units of the body are called segments. Each has a pair of jointed appendages. In others, the segments are fused into a head, thorax, and abdomen. 
	4.	Well-developed nervous system. Arthropods have a brain and a ventral nerve cord. The head bears various types of sense organs, including compound and simple eyes. Many arthropods also have well-developed touch, smell, taste, balance, and hearing capabilities. Arthropods display many complex behaviors and communication skills. 
	5.	Variety of respiratory organs. Marine forms utilize gills; terrestrial forms have book lungs (e.g., spiders) or air tubes called tracheae. Tracheae serve as a rapid way to transport oxygen directly to the cells. The circulatory system is open, with the dorsal heart pumping blood into various sinuses throughout the body. 
	6.	Reduced competition through metamorphosis. Many arthropods undergo a change in form and physiology as a larva becomes an adult. Metamorphosis allows the larva to have a different lifestyle than the adult (Fig. 19.19). For example, larval crabs live among and feed on plankton, while adult crabs are bottom dwellers that catch live prey or scavenge dead organic matter. Among insects such as butterflies, the caterpillar feeds on leafy vegetation, while the adult feeds on nectar.  
Crustaceans, a name derived from their hard, crusty exoskeleton, are a group of largely marine arthropods that include barnacles, shrimps, lobsters, and crabs (Fig. 19.20). There are also some freshwater crustaceans, including the crayfish, and some terrestrial ones, including the sowbug, or pillbug. Although crustacean anatomy is ex-tremely diverse, the head usually bears a pair of compound eyes and five pairs of appendages. The first two pairs of appendages, called antennae and antennules, respectively, lie in front of the mouth and have sensory -functions. The other three pairs are mouthparts used in feeding. In a lobster, the thorax bears five pairs of walking legs. The first walking leg is a pinching claw. The gills are situated above the walking legs. The head and thorax are fused into a cephalothorax, which is covered on the top and sides by a nonsegmented carapace. The abdominal segments, which are largely muscular, are equipped with swimmerets, small paddlelike structures. The last two segments bear the uropods and the telson, which make up a fan-shaped tail to propel the lobster backw-ard (see Fig. 19.18).
Crustaceans play a vital role in the food chain. Tiny crustaceans known as krill are a major source of food for baleen whales, sea birds, and seals. Countries such as Japan are harvesting krill for human use. Copepods and other small crustaceans are primary consumers in marine and aquatic ecosystems. Many species of lobsters, crabs, and shrimp are important in the seafood industry. Some barnacles are destructive to wharfs, piers, and boats. 
Among arthropods, the arachnids include spiders, scorpions, ticks, mites, and harvestmen (“daddy longlegs”) (Fig. 19.21a). Spiders have a narrow waist that separates the cephalothorax from the abdomen. Some spiders inject poisons into their prey and digest their food externally before sucking it into the stomach. Spiders use silk threads for all sorts of purposes, from lining their nests to catching prey. The internal organs of spiders also show how they are adapted to a terrestrial way of life. Malpighian tubules work in conjunction with rectal glands to reabsorb ions and water before a relatively dry nitrogenous waste (uric acid) is excreted. Invaginations of the inner body wall form lamellae (“pages”) of spiders’ so-called book lungs.
Scorpions are the oldest terrestrial arthropods (Fig. 19.21b). Today, they occur in the tropics, subtropics, and temperate regions of North America. They are nocturnal and spend most of the day hidden under a log or a rock. Ticks and mites are parasites. Ticks suck the blood of vertebrates and sometimes transmit diseases, such as Rocky Mountain spotted fever or Lyme disease. Chiggers, the larvae of certain mites, feed on the skin of vertebrates.
The horseshoe crab is grouped with the arachnids because the first pair of appendages are pincerlike structures used for feeding and defense. Horseshoe crabs have pedipalps, which they use as feeding and sensory structures, and four pairs of walking legs (Fig. 19.21c).
Centipedes (Fig. 19.21d), with a pair of appendages on every segment, are carnivorous, while millipedes (Fig. 19.21e), with two pairs of legs on most segments, are herbivorous. These animals are grouped with the insects because they have similar head appendages. 
Insects are so numerous (probably over one million species) and so diverse that the study of this one group is a major specialty in biology called entomology (Fig. 19.22). Some  insects show remarkable behavior adaptations, as exemplified by the social systems of bees, ants, termites, and other colonial insects.
Insects are adapted to an active life on land, although some have secondarily invaded aquatic habitats. The body is divided into a head, a thorax, and an abdomen. The head usually bears a pair of sensory antennae, a pair of compound eyes, and several simple eyes. The mouthparts are adapted to each species’ particular way of life: A grasshopper has mouthparts that chew, and a butterfly has a long tube for siphoning the nectar from flowers. The abdomen contains most of the internal organs; the thorax bears three pairs of legs and the wings—either one or two pairs, or none. Wings enhance an insect’s ability to survive by providing a way of escaping enemies, finding food, facilitating mating, and dispersing the offspring. The exoskeleton of an insect is lighter and contains less chitin than that of many other arthropods. The male has a penis, which passes sperm to the female. The female, as in the grasshopper, may have an ovipositor for laying the fertilized eggs. Some insects, such as butterflies, undergo complete metamorphosis, involving a drastic change in form (see Fig. 19.19).
19.5		Echinoderms and Chordates
The deuterostomes (see page 313) include the echinoderms and the chordates. 
Echinoderms
It may seem surprising that echinoderms (phylum Echinodermata) lack features associated with vertebrates such as humans, and yet are related to chordates (Fig. 19.23). For example, the echinoderms are often radially, not bilaterally, symmetrical. Their larva is a free-swimming filter feeder with bilateral symmetry, but it metamorphoses into a radially symmetrical adult. Also, adult echinoderms do not have a head, brain, or segmentation. The nervous system consists of nerves in a ring around the mouth extending outward radially. Nevertheless, they are classified as deuterostomes as are the chordates.
Echinoderm locomotion depends on a water vascular system. In the sea star, water enters this system through a sieve plate. Eventually it is pumped into many tube feet, expanding them. When the foot touches a surface, the center withdraws, producing suction that causes the foot to adhere to the surface. By alternating the expansion and contraction of its many tube feet, a sea star moves slowly along. 
Echinoderms don’t have a complex respiratory, excretory, or circulatory system. Fluids within the coelomic cavity and a water vascular system carry out many of these functions. For example, gas exchange occurs across the skin gills and the tube feet. Nitrogenous wastes diffuse through the coelomic fluid and the body wall. 
In ecosystems, most echinoderms feed variously on organic matter in the sea or substratum, but sea stars prey upon crustaceans, molluscs, and other invertebrates. From the human perspective, sea stars cause extensive economic loss because they consume oysters and clams before they can be harvested. Fishes and sea otters eat echinoderms, and scientists favor echinoderms for embryological research.
Chordates
To be classified as a chordate (phylum Chordata), an animal must at some time during its life history have the four characteristics depicted in Figure 19.24 and listed here: 
	1.	A dorsal supporting rod, called a notochord. The notochord is located just below the nerve cord toward the back (i.e., dorsal). Vertebrates have an endoskeleton of cartilage or bone, including a vertebral column that has replaced the notochord during development.
	2.	A dorsal tubular nerve cord. Tubular means that the cord contains a canal filled with fluid. In vertebrates, the nerve cord is protected by the vertebrae. Therefore, it is called the spinal cord because the vertebrae form the spine.
	3.	Pharyngeal pouches. These structures are seen only during embryonic development in most vertebrates. In the invertebrate chordates, the fishes, and some amphibian larvae, the pharyngeal pouches become functioning gills. Water passing into the mouth and the pharynx goes through the gill slits, which are supported by gill arches. In terrestrial vertebrates that breathe with lungs, the pouches are modified for various purposes. In humans, the first pair of pouches become the auditory tubes. The second pair become the tonsils, while the third and fourth pairs become the thymus gland and the parathyroids.
	4.	A tail. Because the tail extends beyond the anus, it is called a postanal tail.
The Invertebrate Chordates
There are a few invertebrate chordates in which the notochord is never replaced by the vertebral column. Tunicates (subphylum Urochordata) live on the ocean floor and take their name from a tunic that makes the adults look like thick-walled, squat sacs. They are also called sea squirts because they squirt water from one of their siphons when disturbed (Fig. 19.25a). The tunicate larva is bilaterally symmetrical and has the four chordate characteristics. Metamorphosis produces the sessile adult in which numerous cilia move water into the pharynx and out numerous gill slits, the only chordate characteristic that remains in the adult. 
Lancelets (subphylum Cephalochordata) are marine chordates only a few centimeters long. They have the appearance of a lancet—a small, two-edged surgical knife (Fig. 19.25b). Lancelets are found in the shallow water along most coasts, where they usually lie partly buried in sandy or muddy substrates with only their anterior mouth and gill apparatus exposed. They feed on microscopic particles filtered out of the constant stream of water that enters the mouth and exits through the gill slits. Lancelets retain the four chordate characteristics as adults. In addition, segmentation is present, as witnessed by the fact that the muscles are segmentally arranged and the dorsal tubular nerve cord has periodic branches.
Evolutionary Trends Among the Chordates
Figure 19.26 depicts the evolutionary tree of the chordates and previews the animal groups we will discuss in the remainder of this chapter. The figure also lists at least one main evolutionary trend that distinguishes each group of animals from the preceding ones. The tunicates and lancelets are invertebrate chordates; they don’t have vertebrae. The vertebrates are the fishes, amphibians, reptiles, birds, and mammals. Fishes with an endoskeleton of cartilage and some bone in their scales were the first to have jaws. Early bony fishes had lungs. Amphibians were the first group to clearly have jointed appendages and to invade land. However, the fleshy appendages of lobe-finned fishes from the Devonian era contained bones homologous to those of terrestrial vertebrates. These fishes are believed to be ancestral to the amphibians. Reptiles, birds, and mammals have a means of reproduction suitable to land. During development, an amnion and other extraembryonic membranes are present. These membranes carry out all the functions needed to support the embryo as it develops into a young offspring, capable of feeding on its own.
Fishes: First Jaws and Lungs
The first vertebrates were jawless fishes, which wiggled through the water and sucked up food from the ocean floor. Today, there are three living classes of fishes: jawless fishes, cartilaginous fishes, and bony fishes. The two latter groups have jaws, tooth-bearing bones of the head. Jaws are believed to have evolved from the first pair of gill arches, structures that ordinarily support gills (Fig. 19.27). The presence of jaws permits a predatory way of life.
Living representatives of the jawless fishes (class Agnatha) are cylindrical and up to a meter long. They have smooth, scaleless skin and no jaws or paired fins. The two groups of living jawless fishes are hagfishes and lampreys (Fig. 19.28a). The hagfishes are scavengers, feeding mainly on dead fishes, while some lampreys are parasitic. When parasitic, the round mouth of the lamprey serves as a sucker. The lamprey attaches itself to another fish and taps into its circulatory system. Water cannot move in through the lamprey’s mouth and out over the gills, as is common in all other fishes. Instead, water moves in and out through the gill openings.
Cartilaginous fishes (class Chondrichthyes) are the sharks (Fig. 19.28b), the rays, and the skates, which have skeletons of cartilage instead of bone. The small dogfish shark is often dissected in biology laboratories. One of the most dangerous sharks inhabiting both tropical and temperate waters is the hammerhead shark. The largest sharks, the whale sharks, feed on small fishes and marine invertebrates and do not attack humans. Skates and rays are rather flat fishes that live partly buried in the sand and feed on mussels and clams.
Three well-developed senses enable sharks and rays to detect their prey: (1) They have the ability to sense electric currents in water—even those generated by the muscle movements of animals; (2) they, and all other types of fishes, have a lateral line system, a series of cells that lie within canals along both sides of the body and can sense pressure caused by a fish or another animal swimming nearby; (3) they have a keen sense of smell; the part of the brain associated with this sense is twice as large as the other parts. Sharks can detect about one drop of blood in 115 liters (25 gallons) of water.
Bony fishes (class Osteichthyes) are by far the most numerous and diverse of all the vertebrates. Most of the bony fishes we eat, such as perch, trout, salmon, and haddock, are ray-finned fishes (Fig. 19.28c). Their fins, which are used in balancing and propelling the body, are thin and supported by bony spikes. Ray-finned fishes have various ways of life. Some, such as herring, are filter feeders; others, such as trout, are opportunists; and still others, such as piranhas and barracudas, are predaceous carnivores.
Ray-finned fishes have a swim bladder, which usually serves as a buoyancy organ. By secreting gases into the bladder or by absorbing gases from it, these fishes can change their density, and thus go up or down in the water. The streamlined shape, fins, and muscle action of ray-finned fishes are all suited to locomotion in the water. Their skin is covered by bony scales that protect the body but do not prevent water loss. When ray-finned fishes respire, the gills are kept continuously moist by the passage of water through the mouth and out the gill slits. As the water passes over the gills, oxygen is absorbed by the blood, and carbon dioxide is given off. Ray-finned fishes have a single-circuit circulatory system. The heart is a simple pump, and the blood flows through the chambers, including a nondivided atrium and ventricle, to the gills. Oxygenated blood leaves the gills and goes to the body proper, eventually returning to the heart for recirculation.
Another type of bony fish, called the lobe-finned fishes, evolved into the amphibians (Fig. 19.29). These fishes not only had fleshy appendages that could be adapted to land locomotion, but most also had a lung, which was used for respiration. 
Amphibians: Jointed Vertebrate Limbs
Amphibians (class Amphibia), whose class name means living on both land and in the water, are represented today by frogs, toads, newts, and salamanders. Aside from jointed limbs (Fig. 19.29b), amphibians have other features not seen in bony -fishes: eyelids for keeping their eyes moist, ears adapted to picking up sound waves, and a voice-producing larynx. The brain is larger than that of a fish. Adult amphibians usually have small lungs. Air enters the mouth by way of nostrils, and when the floor of the mouth is raised, air is forced into the relatively small lungs. Respiration is supplemented by gas exchange through the smooth, moist, and glandular skin. The amphibian heart has only three chambers, compared to the four of mammals. Mixed blood is sent to all parts of the body; some is sent to the skin, where it is further oxygenated.
Most members of this group lead an amphibious life—that is, the larval stage lives in the water, and the adult stage lives on the land (see Fig. 19.1). However, the adult usually returns to the water to reproduce.
Reptiles: Amniotic Egg 
Reptiles (class Reptilia) diversified and were most abundant between 245 and 66 mya. These animals included the mammal-like reptiles, the ancestors of today’s living mammals, and the dinosaurs, which became extinct. Some dinosaurs are remembered for their great size. Brachiosaurus, a herbivore, was about 23 meters (75 feet) long and about 17 meters (56 feet) tall. Tyrannosaurus rex, a carnivore, was 5 meters (16 feet) tall when standing on its hind legs. The bipedal stance of some reptiles was preadaptive for the evolution of wings in birds.
The reptiles living today are mainly turtles, crocodiles, snakes, and lizards. The body is covered with hard, keratinized scales, which protect the animal from desiccation and from predators. Another adaptation for a land existence is the manner in which snakes use their tongue as a sense organ (Fig. 19.30). Reptiles have well-developed lungs enclosed by a protective rib cage. The heart has four chambers, but the septum that divides the two halves is incomplete in certain species; therefore, there is some exchange of O2-rich and O2-poor blood.
Perhaps the most outstanding adaptation of the reptiles is that they have a means of reproduction suitable to a land existence. The penis of the male passes sperm directly to the female. Fertilization is internal, and the female lays leathery, flexible, shelled eggs. The amniote egg made development on land possible and eliminated the need for a swimming larval stage during development. The amniote egg provides the developing embryo with atmospheric oxygen, food, and water; removes nitrogenous wastes; and protects the embryo from drying out and from mechanical injury. This is accomplished by the presence of extraembryonic membranes (Fig. 19.31).
Fishes, amphibians, and reptiles are ectothermic, meaning that their body temperature matches the temperature of the external environment. If it is cold externally, they are cold internally; if it is hot externally, they are hot internally. Reptiles regulate their body temperatures by exposing themselves to the sun if they need warmth or by hiding in the shadows if they need cooling off. 
Birds: Feathers
Birds (class Aves) are characterized by the presence of feathers, which are modified reptilian scales. (Perhaps you have noticed the scales on the legs of a chicken.) However, birds lay a hard-shelled amniote egg rather than the leathery egg of reptiles. The exact history of birds is still in dispute, but gathering evidence indicates that birds are closely related to bipedal dinosaurs and that they should be classified as such. 
Nearly every anatomical feature of a bird can be related to its ability to fly (Fig. 19.32). The forelimbs are modified as wings. The hollow, very light bones are laced with air cavities. A horny beak has replaced jaws equipped with teeth, and a slender neck connects the head to a rounded, compact torso. Respiration is efficient since the lobular lungs form anterior and posterior air sacs. The presence of these sacs means that the air moves one way through the lungs, and gases are continuously exchanged across respiratory tissues. Another benefit of air sacs is that they lighten the body and aid flying.
Birds have a four-chambered heart that completely -separates O2-rich blood from O2-poor blood. Birds are endotherms and generate internal heat. Many endotherms can use metabolic heat to maintain a constant internal temperature. This may be associated with their efficient nervous, respiratory, and circulatory systems. Also, their feathers provide insulation. Birds have no bladder and excrete uric acid in a semidry state.
Birds have particularly acute vision and well-developed brains. Their muscle reflexes are excellent. These adaptations are suited to flight. An enlarged portion of the brain seems to be the area responsible for instinctive behavior. A ritualized courtship often precedes mating. Many newly hatched birds require parental care before they are able to fly away and seek food for themselves. A remarkable aspect of bird behavior is the seasonal migration of many species over very long distances. Birds navigate by day and night, whether it’s sunny or cloudy, by using the sun and stars and even the Earth’s magnetic field to guide them.
Traditionally, the classification of birds was particularly based on beak and foot types (Fig. 19.33), and to some extent on habitat and behavior. The various orders include birds of prey with notched beaks and sharp talons; shorebirds with long, slender, probing bills and long, stiltlike legs; woodpeckers with sharp, chisel-like bills and grasping feet; waterfowl with webbed toes and broad bills; penguins with wings modified as flippers; and songbirds with perching feet. Now genetics is used to determine relationships among birds.
Mammals: Hair and Mammary Glands
The first mammals were small, about the size of mice. During all the time the dinosaurs flourished (165 mya), mammals were a minor group that changed little. Some of the earliest mammalian groups are still represented today by the monotremes and marsupials, but they are not abundant. The placental mammals that evolved later went on to live in many habitats.
The two chief characteristics of mammals (class Mammalia) are hair and milk--producing mammary glands. Almost all mammals are endotherms and generate internal heat. Many of the adaptations of mammals are related to temperature control. Hair, for example, provides insulation against heat loss and allows mammals to be active, even in cold weather. 
Mammary glands enable females to feed (nurse) their young without leaving them to find food. Nursing also creates a bond between mother and offspring that helps ensure parental care while the young are helpless. In most mammals, the young are born alive after a period of development in the uterus, a part of the female reproductive system. Internal development shelters the young and allows the female to move actively about while the young are maturing. 
Monotremes are mammals that, like birds, have a cloaca, a terminal region of the digestive tract serving as a common chamber for feces, excretory wastes, and sex cells. They also lay hard-shelled amniote eggs. They are represented by the spiny anteater and the duckbill platypus, both of which live in Australia. The female duckbill platypus lays her eggs in a burrow in the ground. She incubates the eggs, and after hatching, the young lick up milk that seeps from mammary glands on her abdomen. The spiny anteater has a pouch on the belly side formed by swollen mammary glands and longitudinal muscle (Fig. 19.34a). Hatching takes place in this pouch, and the young remain there for about 53 days. Then they stay in a burrow, where the mother periodically visits and nurses them.
The young of marsupials begin their development inside the female’s body, but they are born in a very immature condition. Newborns crawl up into a pouch on their -mother’s abdomen. Inside the pouch, they attach to the nipples of mammary glands and continue to develop. Frequently, more are born than can be accommodated by the number of nipples, and it’s “first come, first served.”
The Virginia opossum is the only marsupial that occurs north of Mexico (Fig. 19.34b). In Australia, however, marsupials underwent adaptive -radiation for several million years without competition. Thus, marsupial mammals are now found mainly in Australia, with some in Central and South America as well. Among the herbivorous marsupials, koalas are tree-climbing browsers (Fig. 19.34c), and kangaroos are grazers. The Tasmanian wolf or tiger, thought to be extinct, was a carnivorous marsupial about the size of a collie dog.
The vast majority of living mammals are placental mammals (Fig. 19.35). In these mammals, the extraembryonic membranes of the reptilian egg (see Fig. 19.31) have been modified for internal development within the uterus of the female. The chorion contributes to the fetal portion of the placenta, while a part of the uterine wall contributes to the maternal portion. Here, nutrients, oxygen, and waste are exchanged between fetal and maternal blood.
Mammals are adapted to life on land and have limbs that allow them to move rapidly. In fact, an evaluation of mammalian features leads us to the obvious conclusion that they lead active lives. The brain is well developed; the lungs are expanded not only by the action of the rib cage but also by the contraction of the diaphragm, a horizontal muscle that divides the thoracic cavity from the abdominal cavity; and the heart has four chambers. The internal temperature is constant, and hair, when abundant, helps insulate the body.
The mammalian brain is enlarged due to the expansion of the cerebral hemispheres that control the rest of the brain. The brain is not fully developed until after birth, and young learn to take care of themselves during a period of dependency on their parents.
Mammals can be distinguished by their methods of obtaining food and their mode of -locomotion. For example, bats have membranous wings supported by digits; horses have long, hoofed legs; and whales have paddlelike forelimbs. The specific shape and size of the teeth may be associated with whether the mammal is a herbivore (eats vegetation), a carnivore (eats meat), or an omnivore (eats both meat and vegetation). For example, mice have continuously growing incisors; horses have large, grinding molars; and dogs have long canine teeth.
19.6		Human Evolution
The evolutionary tree in Figure 19.36 shows that all primates share one common ancestor and that the other types of primates diverged from the human line of descent (called a lineage) over time. The order Primates has two suborders. The prosimians include the lemurs, tarsiers, and lorises. The anthropoids include the monkeys, apes, and humans. This classification tells us that humans are more closely related to the monkeys and apes than they are to the prosimians.
Primates are adapted to an arboreal life—meaning, for living in trees. Primate limbs are mobile, and the hands and feet have five digits each. Many primates have both an opposable big toe and a thumb—that is, the big toe or thumb can touch each of the other toes or fingers. (Humans don’t have an opposable big toe, but the thumb is opposable, resulting in a grip that is both powerful and precise.) The opposable thumb allows a primate to easily reach out and bring food such as fruit to the mouth. When locomoting, primates grasp and release tree limbs freely because nails have replaced claws. 
The evolutionary trend among primates is toward a larger and more complex brain—the brain size is smallest in prosimians and largest in modern humans. In humans, the cerebral cortex has many association areas and expands so much that it becomes extensively folded. The portion of the brain devoted to smell gets smaller, and the portions devoted to sight increase in size and complexity during primate evolution. Also, more and more of the brain is involved in controlling and processing information received from the hands and the thumb. The result is good hand-eye coordination in humans.
Notice that prosimians were the first type of primate to diverge from the human line of descent, and African apes were the last group to diverge from our line of descent. The evolutionary tree also indicates that humans are most closely related to African apes. One of the most unfortunate misconceptions concerning human evolution is that Darwin and others suggested that humans evolved from apes. On the contrary, humans and apes are believed to share a common apelike ancestor. Today’s apes are our distant cousins, and we couldn’t have evolved from our cousins because we are contemporaries—living on Earth at the same time. Presently, researchers believe that the last common ancestor for African apes and humans lived about 7 mya.
Evolution of Hominids
All of the fossils shown in Figure 19.37 are hominids. This illustration tells when these hominids lived and emphasizes the type of face and brain size. However, to be hominid, it is only necessary that the fossil have an anatomy suitable for standing erect and walking on two feet, a characteristic called -bipedalism (Fig. 19.37). 
Australopithecines
It’s possible that one of the australopithecines, a group of hominids that evolved and diversified in Africa about 4 mya, is a direct ancestor of humans. More than 20 years ago, a team led by Donald Johanson unearthed nearly 250 fossils of a hominid called Australopithecus afarensis. A now-famous female skeleton dated at 3.18 mya is known worldwide by its field name, Lucy. (The name derives from the Beatles song “Lucy in the Sky with Diamonds.”) Although her brain was quite small (400 cc), the shapes and relative proportions of Lucy’s limbs indicate that she stood upright and walked bipedally (Fig. 19.38a). Even better evidence of bipedal locomotion comes from a trail of footprints dated about 3.7 mya. The larger prints are double, as though a smaller-sized individual was stepping in the footfalls of another—and there are additional small prints off to the side, within hand-holding distance (Fig. 19.38b).
Since the australopithecines were apelike above the waist (small brain) and humanlike below the waist (walked erect), it seems that human characteristics did not evolve all at one time. The term mosaic evolution is applied when different body parts change at different rates and therefore at different times.
Homo habilis
Homo habilis, dated between 2.0 and 1.9 mya, may be ancestral to modern humans. Some of these fossils have a brain size as large as 775 cc, which is about 45% larger than the brain of A. afarensis. The cheek teeth are smaller than those of any of the australopithecines. Therefore, it is likely that this early Homo was omnivorous and ate meat in addition to plant material. Bones at the campsites of H. habilis bear cut marks, indicating that they used tools to strip the meat from bones.
The stone tools made by H. habilis, whose name means “handyman,” are rather crude (see Fig. 19.37). It’s possible that these are the cores from which these hominids took flakes sharp enough to scrape away hide, cut tendons, and easily remove meat from bones.
The skulls of H. habilis suggest that the portions of the brain associated with speech areas were enlarged. We can speculate that the ability to speak may have led to hunting cooperatively. Some members of the group may have remained plant gatherers, and if so, both hunters and gatherers most likely ate together and shared their food. In this way, society and culture could have begun. Culture, which encompasses human behavior and products (such as technology and the arts), is dependent upon the capacity to speak and transmit knowledge. We can further speculate that the advantages of a culture to H. habilis may have hastened the extinction of the australopithecines.
Homo erectus
Homo erectus and like fossils are found in Africa, Asia, and Europe and dated between 1.9 and 0.3 mya. Although all fossils assigned the name H. erectus are similar in appearance, there is enough discrepancy to suggest that several different species have been included in this group. Compared to H. habilis, 
H. erectus had a larger brain (about 1,000 cc) and a flatter face. The recovery of an almost complete skeleton of a 10-year-old boy indicates that H. erectus was much taller than the hominids discussed thus far. Males were 1.8 meters tall (about 6 feet), and females were 1.55 meters (approaching 5 feet). Indeed, these hominids were erect and most likely had a striding gait like that of modern humans. The robust and most likely heavily muscled skeleton still retained some australopithecine features. Even so, the size of the birth canal indicates that infants were born in an immature state that required an extended period of care.
It is believed that H. erectus first appeared in Africa and then migrated into Asia and Europe. At one time, the migration was thought to have occurred about 1 mya, but -recently H. erectus fossil remains in Java and the Republic of Georgia have been dated at 1.9 and 1.6 mya, respectively. These remains push the evolution of H. erectus in Africa to an earlier date than has yet been determined. In any case, such an extensive population movement is a first in the history of humankind and a tribute to the intellectual and physical skills of the species.
H. erectus was the first hominid to use fire, and it also fashioned more advanced tools than earlier Homo species. These hominids used heavy, teardrop-shaped axes and cleavers, as well as flakes that were probably used for cutting and scraping. Some investigators believe H. erectus was a systematic hunter that brought kills to the same site over and over again. In one location, researchers have found over 40,000 bones and 2,647 stones. These sites could have been “home bases” where social interaction occurred and a prolonged childhood allowed time for learning. Perhaps a language evolved and a culture more like our own developed.
Evolution of Modern Humans
Most researchers believe that Homo sapiens (modern humans) evolved from H. erectus, but they differ as to the details. The hypothesis that H. sapiens evolved separately from H. erectus in Asia, Africa, and Europe is called the -multiregional -continuity hypothesis (Fig. 19.39a). This hypothesis proposes that evolution to modern humans was essentially similar in several different places. If so, each region should show a continuity of its own anatomical characteristics from the time when H. erectus first arrived.
Opponents argue that it seems highly unlikely that evolution would have produced essentially the same result in these different places. They suggest, instead, the out-of-Africa hypothesis, which proposes that H. sapiens evolved from H. erectus only in Africa, and thereafter H. sapiens migrated to Europe and Asia about 100,000 years bp (before present) (Fig. 19.39b). If so, fossils dated 200,000 bp and 100,000 bp are expected to be markedly different from each other.
According to which hypothesis would modern humans be most genetically alike? The multiregional continuity hypothesis states that human populations have been evolving separately for a long time; therefore, genetic differences would be expected between groups. According to the out-of-Africa hypothesis, we are all descended from a few individuals from about 100,000 years bp. Therefore, the out-of-Africa hypothesis suggests that we are more genetically similar.
A few years ago, a study attempted to show that all the people of Europe (and the world, for that matter) have essentially the same mitochondrial DNA. Called the “mitochondrial Eve” hypothesis by the press (note that this is a misnomer because no single ancestor is proposed), the statistics that calculated the date of the African migration were found to be flawed. Still, the raw data—which indicate a close genetic relationship among all Europeans—support the out-of-Africa hypothesis.
These opposing hypotheses have sparked many other innovative studies. The final conclusions are still being determined.
Neandertals
Neandertals (H. neandertalensis) take their name from Germany’s Neander Valley, where one of the first Neandertal skeletons, dated some 200,000 years ago, was discovered. It’s possible that the Neandertals were already present in Eurasia when modern humans represented by Cro-Magnons (discussed next) arrived on the scene. In that case, competition with the Cro-Magnons may have made the Neandertals become extinct. The Neandertals had massive brow ridges, and their nose, jaws, and teeth protruded far forward. The forehead was low and sloping, and the lower jaw lacked a chin. The Neandertals were heavily muscled, especially in the shoulders and neck. The bones of their limbs were shorter and thicker than those of modern humans. The Neandertals lived in Europe and Asia during the last Ice Age, and their sturdy build could have helped conserve heat.
The Neandertals give evidence of being culturally advanced. Most lived in caves, but those living in the open may have built houses. They manufactured a variety of stone tools, including spear points, which could have been used for hunting, and scrapers and knives, which would have helped with food preparation. They most likely successfully hunted bears, woolly mammoths, rhinoceroses, reindeer, and other contemporary animals. They buried their dead with flowers and tools and may have had a religion.
Cro-Magnons 
Cro-Magnons are the oldest fossils to be designated H. sapiens. Cro-Magnons, who are named after a fossil location in France, had a thoroughly modern appearance. They made advanced tools, including compound tools, as when stone flakes were fitted to a wooden handle. They may have been the first to make knifelike blades and to throw spears, enabling them to kill animals from a distance. They were such accomplished hunters that some researchers believe they were responsible for the extinction of many larger mammals, such as the giant sloth, the mammoth, the saber-toothed tiger, and the giant ox, during the late Pleistocene epoch.
Cro-Magnons hunted cooperatively, and were perhaps the first to have a language. They are believed to have lived in small groups, with the men hunting by day while the women remained at home with the children, gathering and processing food items. Probably, the women also were engaged in maintenance tasks. The Cro-Magnon culture included art. They sculpted small figurines out of reindeer bones and antlers. They also painted beautiful drawings of animals, some of which have survived on cave walls in Spain and France (Fig. 19.40).
The Chapter in Review
Summary
19.1		Evolution of Animals
Animals are motile, multicellular heterotrophs that ingest their food.
Evolutionary History of Animals
Animals representative of today’s phyla appear in the fossil record during the Cambrian period. This burst of diversification is called the Cambrian explosion.
Evolutionary Tree of Animals
An evolutionary tree, based on seven innovations, depicts the seven evolutionary trends that occurred as animals evolved:
	•	Multicellularity
	•	True tissues
	•	Bilateral symmetry
	•	Body cavity
	•	Coelomates
	•	Segmentation
	•	Jointed appendages
19.2		Introducing the Invertebrates
Sponges are multicellular (lack tissues) and asymmetrical.
Cnidarians have two tissue layers, are radially symmetrical, have a saclike digestive cavity, and possess stinging cells and nematocysts.
Flatworms have ectoderm, endoderm, and mesoderm but no coelom. They are bilaterally symmetrical and have a saclike digestive cavity.
Roundworms have a pseudocoelom and a complete digestive tract.
19.3		Protostomes and Deuterostomes Compared
On the basis of embryological data:
	•	Molluscs, annelids, and arthropods are protostomes; the first embryonic opening becomes the mouth, and the coelom develops by a splitting of mesoderm.
	•	Echinoderms and chordates are deuterostomes; the second embryonic opening becomes the mouth, and the coelom forms by outpocketing of the primitive gut.
19.4		Molluscs, Annelids, and Arthropods
Molluscs
The body of a mollusc typically contains a visceral mass, a mantle, and a foot.
GastropodsSnails, representatives of this group, have a flat foot, a one-part shell, and a mantle cavity that carries on gas exchange.
CephalopodsOctopuses and squids display marked cephalization, move rapidly by jet propulsion, and have a closed circulatory system.
BivalvesBivalves such as clams have a hatchet foot and a two-part shell, and are filter feeders.
Annelids: Segmented Worms
Annelids are segmented worms; segmentation is seen both externally and internally.
Marine wormsPolychaetes are worms that have many setae. A clam worm is a predaceous marine worm with a defined head region.
EarthwormsEarthworms are oligochaetes that scavenge for food in the soil and do not have a well-defined head region.
LeechesLeeches are annelids that feed by sucking blood.
Arthropods: Jointed Appendages
Arthropods are the most varied and numerous of animals. Their success is largely attributable to a flexible exoskeleton and specialized body regions.
Crustaceans have a head that bears compound eyes, antennae, and mouthparts. Five pairs of walking legs are present.
Arachnids have four pairs of walking legs attached to a cephalothorax. Horseshoe crabs are marine arachnids. Spiders live on land and spin silk, which they use to capture prey as well as for other purposes. 
Insects have three pairs of legs attached to the thorax. Insects have adaptations to a terrestrial life, such as wings for flying.
19.5		Echinoderms and Chordates
Echinoderms
Echinoderms have radial symmetry as adults (not as larvae) and endoskeletal spines. Typical echinoderms have tiny skin gills, a central nerve ring with branches, and a water vascular system for locomotion, as exemplified by the sea star.
Chordates
Chordates (tunicates, lancelets, and vertebrates) have a notochord, a dorsal tubular nerve cord, pharyngeal pouches, and a postanal tail at some time in their life history.
Invertebrate ChordatesAdult tunicates lack chordate characteristics except gill slits, but adult lancelets have the four chordate characteristics and show obvious segmentation.
Vertebrate ChordatesVertebrate chordates include the fishes, amphibians, reptiles, birds, and mammals.
Fishes
•	The first vertebrates, represented by hagfishes and lampreys, lacked jaws and fins.
•	Cartilaginous fishes, represented by sharks and rays, have jaws and a skeleton made of cartilage.
•	Bony fishes have jaws and fins supported by bony spikes; the bony fishes include those that are ray-finned and a few that are lobe-finned. Some of the lobe-finned fishes have lungs.
Amphibians (frogs, toads, newts, and salamanders) evolved from the lobe-finned fishes and have two pairs of jointed vertebrate limbs. Frogs usually return to the water to reproduce; frog tadpoles metamorphose into terrestrial adults.
Reptiles (today’s snakes, lizards, turtles, and crocodiles) lay a shelled egg, which contains extraembryonic membranes, including an amnion that allows them to reproduce on land.
Birds are feathered, which helps them maintain a constant body temperature. They are adapted for flight; their bones are hollow with air cavities; lungs form air sacs that allow one-way ventilation; and they have well-developed sense organs.
Mammals have hair and mammary glands. The former helps them maintain a constant body temperature, and the latter allows them to nurse their young.
•	Monotremes lay eggs.
•	Marsupials have a pouch in which the newborn matures.
•	Placental mammals, which are far more varied and numerous, retain offspring inside the uterus until birth.
19.6		Human Evolution

Primates are mammals adapted to living in trees. During the evolution of primates, various groups diverged in a particular sequence from the line of descent leading to today’s primates. Prosimians (tarsiers and lemurs) diverged first, followed by the monkeys, then the apes, and then humans. Molecular biologists tell us we are most closely related to the African apes, with which we share a common ancestor about 7 mya.
Evolution of Hominids
Human evolution occurred in Africa.

 •	
The most famous australopithecine is Lucy (3.18 mya) whose brain was small but who walked bipedally.

•	
Homo habilis, present about 2 mya, is certain to have made tools.

•	
Homo erectus, with a brain capacity of 1,000 cc and a striding gait, was the first to migrate out of Africa.

Evolution of Modern Humans
Two contradicting hypotheses have been suggested about the origin of modern humans:
Multiregional Continuity HypothesisModern humans originated separately in Asia, in Europe, and in Africa.
Out-of-Africa HypothesisModern humans originated in Africa and, after migrating into Europe and Asia, replaced the other Homo species (including the Neandertals) found there. Cro-Magnon was the first hominid to be considered Homo sapiens.
Thinking Scientifically
	1.	Recently, three fossil skulls of Homo sapiens, dating to about 160,000 years bp, were discovered in eastern Africa. These skulls fill a gap between the 100,000-year-old H. sapiens skulls found in Africa and Israel and 500,000-year-old skulls of archaic H. sapiens found in Ethiopia. Supporters of the out-of-Africa hypothesis argue that this discovery strengthens their position by documenting the succession of human ancestors from 6 mya through this most recent group. Does this finding prove that the out-of-Africa hypothesis is correct? If not, what fossil evidence might yet be found to support the multiregional continuity hypothesis? 
	2.	Think of the animals in this chapter that are radially symmetrical (cnidarians, many adult echinoderms). How is their lifestyle different from that of bilaterally symmetrical animals? How does their body plan complement their lifestyle?
Testing Yourself
Choose the best answer for each question.
	1.	Which of the following is the least likely explanation for the Cambrian explosion?
	a. The development of skeletons may have provided protection against predators.
	b. Mutation rates were higher than normal.
	c. Oxygen levels were on the rise.
	d. Skeletons survived as fossils.
	2.	In the following diagram, label the major regions of the bodies of acoelomates, pseudocoelomates, and coelomates.
	3.	Sponges are ancestors of
	a. cnidarians.
	b. protostomes.
	c. deuterostomes.
	d. None of these are correct.
For questions 4–10, identify the group(s) to which each feature belongs. Each answer may be used more than once. Each question may have more than one answer.
Key:
	a. flatworms
	b. roundworms
	c. molluscs
	d. annelids
	e. arthropods
	f. echinoderms
	g. chordates
	4.	Most have an endoskeleton.
	5.	Typically bilaterally symmetrical.
	6.	Nonsegmented. 
	7.	Lack any kind of coelom. 
	8.	Chitin exoskeleton. 
	9.	Contain a mantle.
	10.	Move by pumping water. 
	11.	Insects have wings and three pairs of legs attached to the
	a. abdomen.
	b. thorax.
	c. head.
	d. midsection.
	12.	Cartilaginous fishes detect their prey by sensing
	a. electrical currents.
	b. odors.
	c. pressure changes.
	d. More than one of these are correct.
	e. All of these are correct.
	13.	Which of the following is not a feature of mammals?
	a. hair
	b. milk-producing glands
	c. ectothermic
	d. four-chambered heart
	e. diaphragm to help expand lungs
	14.	This species was probably the first to have the use of fire.
	a. Homo habilis
	b. Homo erectus
	c. Homo sapiens
	d. Australopithecus robustus
	e. Australopithecus afarensis
	15.	Which of these is matched correctly?
	a. Australopithecus afarensis—bipedal but small brain
	b. Homo habilis—made tools and migrated often
	c. Homo erectus—had fire and migrated out of Europe to Africa
	d. Homo sapiens—projecting face and had culture
	e. Both a and c are correct.
	16.	Cnidarians are considered to be organized at the tissue level because they contain
	a. ectoderm and endoderm.
	b. ectoderm.
	c. ectoderm and mesoderm.
	d. endoderm and mesoderm.
	e. mesoderm.
	17.	A cnidocyte is
	a. a digestive cell.
	b. a reproductive cell.
	c. a stinging cell.
	d. an excretory cell.
	18.	Which of the following is not a feature of a coelomate?
	a. radial symmetry
	b. three germ layers
	c. complete digestive tract body plan
	d. organ level of organization
	19.	A mollusc’s shell is secreted by the
	a. foot.
	b. head.
	c. visceral mass.
	d. mantle.
	20.	The type of mollusc that produces tentacles is a
	a. gastropod.
	b. bivalve.
	c. univalve.
	d. cephalopod.
	21.	A feature of annelids is
	a. a segmented body.
	b. acoelomate.
	c. a sac body plan.
	d. radial symmetry.
	22.	Which of the following is not a feature of an insect?
	a. compound eyes
	b. eight legs
	c. antennae
	d. an exoskeleton
	e. jointed legs
	23.	Which of the following undergoes metamorphosis?
	a. grasshopper
	b. crayfish
	c. earthworm
	d. More than one of these are correct.
	24.	Which of the following is not an arachnid?
	a. spider
	b. tick
	c. mite
	d. scorpion
	e. beetle
	25.	Unlike bony fishes, amphibians have
	a. ears.
	b. jaws.
	c. a circulatory system.
	d. a heart.
	26.	Which of the following is not an adaptation for flight in birds?
	a. air sacs
	b. modified forelimbs
	c. bones with air cavities
	d. acute vision
	e. well-developed bladder
	27.	Examples of monotremes include the
	a. spiny anteater and duckbill platypus.
	b. opossum and koala.
	c. badger and skunk.
	d. porcupine and armadillo.
	28.	Mammals are distinguished based on
	a. size and hair type.
	b. mode of reproduction.
	c. number of limbs and method of caring for young.
	d. number of mammary glands and number of offspring.
	29.	The first humanlike feature to evolve in the hominids was
	a. a large brain.
	b. massive jaws.
	c. a slender body.
	d. bipedal locomotion.
	30.	In H. habilis, enlargement of the portions of the brain associated with speech probably led to 
	a. cooperative hunting.
	b. the sharing of food.
	c. the development of culture.
	d. All of these are correct.
	31.	Mitochondrial DNA data support which hypothesis for the evolution of humans?
	a. multiregional continuity
	b. out-of-Africa
	32.	The first Homo species to use art appears to be
	a. H. neandertalensis.
	b. H. erectus.
	c. H. habilis.
	d. H. sapiens.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
People who approve of laboratory research involving animals point out that even today it would be difficult to develop new vaccines and medicines against infectious diseases, new surgical techniques for saving human lives, or new treatments for spinal cord injuries without the use of animals. Even so, most scientists favor what are now called the “three Rs”: (1) replacement of animals by in vitro, or test-tube, methods whenever possible; (2) reduction of the number of animals used in experiments; and (3) refinement of experiments to cause less suffering to animals. 
F. Barbara Orlans of the Kennedy Institute of Ethics at Georgetown University says, “It is possible to be both pro research and pro reform.” She feels that animal activists need to accept that sometimes animal research is beneficial to humans, and all scientists need to consider the ethical dilemmas that arise when animals are used for laboratory research. Do you approve of this compromise? 
Understanding the Terms
acoelomate308
amphibian323
annelid315
anthropoid328
arachnid317
arthropod316
australopithecine330
bilateral symmetry308
bipedalism329
bird324
bivalve314
bony fishes322
Cambrian explosion307
cartilaginous fishes322
cephalization308
cephalopod314
chitin316
chordate320
cnidarian310
coelom308
Cro-Magnon332
crustacean316
culture330
deuterostome313
echinoderm319
ectothermic324
endoskeleton307
endothermic325
exoskeleton307
filter feeder310
flatworm311
gastropod314
hermaphrodite312
hominid329
Homo erectus330
Homo habilis330
Homo sapiens331
insect318
jawless fishes322
lancelet320
leech315
lineage328
lobe-finned fishes323
lung323
mammal326
marsupial326
molting316
monotreme326
mosaic evolution330
multiregional continuity
hypothesis331
Neandertal332
nematocyst311
nephridium315
notochord320
out-of-Africa hypothesis331
placental mammal327
planarian311
prosimian328
protostome313
pseudocoelom308
radial symmetry308
ray-finned fishes322
reptile324
roundworm312
segmentation309
sponge310
tunicate320
vertebrate320
Match the terms to these definitions:
a.	_______________	Localization of the brain and sensory organs at the anterior end of an animal. 
b.	_______________	Body cavity enclosed by mesoderm.
c.	_______________	Possesses both male and female sex organs. 
d.	_______________	Longitudinal cut through the body at any point gives two equal halves.
e.	_______________	Generate internal heat. 

Arboreal
Both a gull, which flies in the air, and a penguin, which doesn’t, are birds. 
Both a cat and a duckbill platypus, which has a bill like a duck, are mammals. 
Check Your Progress
	1.	Compare and contrast nutrition in animals with nutrition 
in fungi.
	2.	
Compare and contrast nutrition in animals with nutrition 
in plants.
Answers:1. Both animals and fungi are chemoheterotrophs. Fungi digest their food externally, while animals digest their food internally.2. Animals are chemoheterotrophs, while plants are photosynthesizers.
Figure 19.2Sea life in the Cambrian period.
The animals depicted here are found as fossils in the Burgess Shale, a formation in the Rocky Mountains of British Columbia. Some lineages represented by these animals are still evolving today; others have become extinct.
Figure 19.4Radial versus bilateral symmetry.
a. With radial symmetry, two mirror images are obtained no matter how the animal is sliced longitudinally. Radially symmetrical animals tend to stay in one place and reach out in all directions to get their food. b. With bilateral symmetry, mirror images are obtained only if the animal is sliced down the middle. Bilaterally symmetrical animals tend to actively go after their food.  
Figure 19.5Type of body cavity. 
a. Flatworms don’t have a body cavity; they are acoelomates, and mesoderm fills the space between ectoderm and endoderm. 
b. Roundworms are pseudocoelomates; they have a body cavity, and mesoderm lies next to the ectoderm but not the endoderm. c. Other animals are coelomates, and mesoderm lines the entire body cavity. 
Figure 19.6Proposed evolutionary tree of animals. 
This proposed evolutionary tree is based on RNA sequence data. Compared to the traditional tree, many more phyla are considered protostomes and they are divided into two unique groups. The annelids (segmented worms, such as the earthworm) are placed in one group, and the arthropods are placed in another, even though both are segmented animals. 
Figure 19.3Evolution of animals.
All animals are believed to be descended from a type of protist; however, sponges may have evolved from a protist separately from the rest of the animals.
Figure 19.9More 
cnidarians. 
a. The Portuguese man-of-war is a colony of polyp and medusa types of individuals. One polyp becomes a gas-filled float, and the other polyps are specialized for feeding. 
b. The calcium carbonate skeletons of corals form the coral reefs, areas of biological diversity. c. In jellyfishes, the medusa is the primary stage of the life cycle, and a polyp form remains quite small and inconspicuous.
Figure 19.10Anatomy of a planarian.
a. This drawing shows that flatworms are bilaterally symmetrical and have a head region with eyespots. b. The excretory system with flame cells is shown in detail. c. The nervous system has a ladderlike appearance. d. The reproductive system (shown in brown) has both male and female organs, and the digestive system (shown in pink) has a single opening. When the pharynx is extended as shown in (a), a planarian sucks food up into a gastrovascular cavity that branches throughout 
its body.
Figure 19.7Sponge anatomy.
Water enters a sponge through pores and circulates past collar cells before exiting at the mouth. Collar cells digest small particles that become trapped by their appendages, flagella in the microvilli of their collar. Amoeboid cells transport nutrients from cell to cell.
Figure 19.8
Cnidarians.
Hydras (a) and sea anemones (b) are solitary polyps that use tentacles laden with stinging cells to capture their food.
Check Your Progress
	1.	Explain how sponges obtain their food.
	2.	
Explain how cnidarians capture their food.
	3.	Compare and contrast planarians with tapeworms and flukes.
	4.	List the features of roundworms that are not present in flatworms, cnidarians, or sponges.
Answers:1. Sponges are filter feeders, straining food particles out of the water; the food particles are then engulfed by cells that line their central cavity.2. Cnidarians use stinging cells called cnidocytes to sting their prey, trapping or paralyzing them so they can be drawn into the gastrovascular cavity.3. All are flatworms, which are acoelomates with bilateral symmetry. Planarians are free-living, while tapeworms and flukes are parasitic.
4. Roundworms have a complete digestive tract and a body cavity.
Figure 19.13Development in protostomes and 
deuterostomes.
a. In protostomes, the mesoderm splits, creating a coelom; the blastopore becomes the mouth. b. In deuterostomes, the primitive gut outpockets to form the coelom; the blastopore becomes the anus.
Figure 19.11Parasitic flatworms.
a. The anterior end of a tapeworm has hooks and suckers for attachment to the intestinal wall. b. Sexes are separate in blood flukes, which cause schistosomiasis. 
Figure 19.12Roundworm anatomy.
a. The roundworm Ascaris. b. Roundworms such as Ascaris have a pseudocoelom and a complete digestive tract with a mouth and an anus. The sexes are separate; this is a male roundworm. c. The larvae of the roundworm Trichinella penetrate striated muscle fibers, where they coil in a sheath formed from the muscle fiber.
Figure 19.16Earthworm anatomy.
a. Internal anatomy of the anterior part of an earthworm. b. Cross section of an earthworm.
Figure 19.17Other annelids.
A polychaete can be predacious like this marine clam worm (a) or live in a tube like this fan worm called the Christmas tree worm (b). 
c. The medicinal leech, also an annelid, is sometimes used to remove blood from tissues after surgery.
Figure 19.14Body plan of molluscs.
Molluscs have a three-part body: a. a muscular foot, a visceral mass, and a mantle. b. In the mouth, the radula is a tonguelike organ that bears rows of tiny, backward-pointing teeth. 
Figure 19.15Three classes of molluscs.
Top row: Snails (left) and nudibranchs (right) are gastropods. Middle row: Octopuses (left) and nautiluses (right) are cephalopods. Bottom row: Scallops (left) and mussels (right) are bivalves. 
Figure 19.20Crustacean diversity.
a. A copepod uses its long antennae for floating and its feathery maxillae for filter feeding. Shrimp (b) and crabs (c) are decapods—they have five pairs of walking legs. Shrimp resemble crayfish more closely than they do crabs, which have a reduced abdomen. Marine shrimp feed on copepods. d. The gooseneck barnacle is attached to an object by a long stalk. Barnacles have no abdomen and a reduced head; the thoracic legs project through a shell to filter feed. Barnacles often live on man-made objects such as ships, buoys, and cables. 
Figure 19.21More arthropods. 
a. The black widow spider is a poisonous spider that spins a web. b. Scorpions have large pincers in front and along abdomen, which ends with a stinger containing venom. c. Horseshoe crabs are common along the North American east coast. 
d. A centipede has a pair of appendages on almost every segment. e. A millipede has two pairs of legs on most segments.
Figure 19.18Exoskeleton and jointed appendages.
Arthropods, such as a lobster, have various appendages attached to the head region of a cephalothorax and five pairs of walking legs attached to the thorax region. Appendages called swimmerets, used in reproduction and swimming, are attached to the abdomen. The uropods and telson make up a fan-shaped tail.
Figure 19.19Monarch butterfly metamorphosis.
a. A caterpillar (larva) eats and grows. b. After the larva goes through several molts, it builds a cocoon around itself and becomes a pupa. c. Inside the pupa, the larva undergoes changes in organ structure to become an adult. d, e. The adult butterfly emerges from the cocoon and reproduces, and the cycle begins again.
Figure 19.23Echinoderm diversity.
a. Anatomy of a sea star. b. Sea lilies are immobile, but feather stars can move about. They usually cling to coral or sponges where they feed on plankton. c. Sea cucumbers have a long, leathery body that resembles a cucumber, except for the feeding tentacles about the mouth. d. Brittle stars have a central disk from which long, flexible arms radiate. e. Sea urchins and sand dollars have spines for locomotion, defense, and burrowing.
Figure 19.22Insect diversity.
Check Your Progress
Check Your Progress
	1.	What are unique features of echinoderms?
	2.	
List the four distinguishing features of chordates.
Answers:1. Echinoderms have radial symmetry as adults and a water vascular system with tube feet for locomotion.2. Chordates have a notochord, dorsal tubular nerve cord, pharyngeal pouches, and postanal tail.
Figure 19.26Evolutionary tree of chordates.
Each of the numbered innovations is shared by the classes beyond that point.
Figure 19.24The four chordate characteristics.
Figure 19.25Invertebrate chordates.
a. Tunicates (sea squirts) have numerous gill slits, the only chordate characteristic that remains in the adult. b. Lancelets have all four chordate characteristics as adults.
Check Your Progress
	1.	Explain the evolutionary significance of the development of a jaw.
	2.	
Among amphibians, what features in particular are adaptations to life on land?
Answers:1. The jaw allowed vertebrates to become predators.2. Amphibians have jointed limbs, eyelids for keeping their eyes moist, ears for picking up sound waves, a voice-producing larynx, and usually lungs.
Figure 19.29Evolution of amphibians.
a. A lobe-finned fish compared with an amphibian. A shift in the position of the bones in the forelimbs and hind limbs lifts and supports the body. b. Newt (shown here) and frogs (see Fig. 19.1) are types of living amphibians.
Figure 19.27Evolution of jaws.
Jaws evolved from the anterior gill arches of ancient jawless fishes.
Figure 19.28
Diversity of fishes.
a. The lamprey is a jawless fish. Note the toothed oral disk. b. The shark is a cartilaginous fish. c. The soldierfish, a bony fish, has the typical appearance of a ray-finned fish.  
Figure 19.32Bird flight.
Birds fly by flapping their wings. Bird flight requires an airstream and a powerful wing downstroke for lift, a force at right angles to the airstream.
Figure 19.33Bird beaks.
a. A cardinal’s beak allows it to crack tough seeds. b. A bald eagle’s beak allows it to tear prey apart. c. A flamingo’s beak strains food from the water with bristles that fringe the mandibles.
Figure 19.30Reptiles.
Snakes wave a forked tongue in the air to collect chemical mole-cules, which are then brought back into the mouth and delivered to an organ in the floor of the mouth. Analyzed chemicals help the snake trail its prey, recognize a predator, or find a mate.
Figure 19.31Reproduction on land. 
a. A baby American crocodile hatching out of its shell. Note that the shell is leathery and flexible, not brittle like a bird’s egg. b. Inside the egg, the embryo is surrounded by extraembryonic membranes. The chorion aids gas exchange, the yolk sac provides nutrients, the allantois stores waste, and the amnion encloses a fluid that prevents drying out and provides protection. 
Figure 19.35Placental mammals.
Placental mammals have adapted to various ways of life. a. Deer are herbivores that live in forests. b. Lions are carnivores on the African plain. c. Monkeys inhabit tropical forests. d. Whales are sea-dwelling placental mammals.
Figure 19.34Monotremes and marsupials.
a. The spiny anteater is a monotreme that lives in Australia. b. The opossum is the only marsupial in the United States. The Virginia opossum is found in a variety of habitats. c. The koala is an Australian marsupial that lives in trees.
Check Your Progress
	1.	List the three body parts of a mollusc.
	2.	List the main features of annelids.
	3.	List the five major features that have helped to make arthropods successful.
Answers:1. The mollusc’s body has a visceral mass, a foot, and a mantle.
2. Annelids are segmented, with a complete digestive tract body plan. They contain a digestive system, clo-sed circulatory system, nervous system, and excretory system.3. Rigid, jointed exoskeleton; segmented body; well-developed nervous system; variety of respiratory organs; many go through metamorphosis.

	1.	Distinguish between the three types of mammals.
	2.	
List the two main features of mammals.
Answers:1. Monotremes lay eggs, marsupials have a pouch where newborn offspring finish development, and the offspring of placental mammals develop internally in the uterus.2. Mammary glands and hair are the two main features of mammals.
Check Your Progress
	1.	Describe how reptiles are adapted to reproduce on land.
	2.	
List the anatomical features of birds that help them fly.
	3.	
List the two main features of mammals.
Answers:1. Fertilization is internal in reptiles and results in the production of a leathery amniote egg.2. Birds’ forelimbs are modified as wings; their bones are usually hollow with air cavities; air sacs involved in respiration also lighten the body; and they have a well-developed nervous system and muscles.
3. Mammary glands and hair are the two main features of mammals.
Check Your Progress
	1.	What groups of animals are primates? 
	2.	Did humans evolve from apes? 
Answers:1. Prosimians, monkeys, apes, and humans are primates.
2. No, humans and apes share a common ancestor.
Figure 19.36Evolutionary tree of primates.
The ancestor to all primates climbed into one of the first fruit-bearing forests about 66 mya. The descendants of this ancestor adapted to the new way of life and developed such traits as a shortened snout and nails instead of claws. The time when the humans diverged from the ape line of descent is largely known from the fossil record. A common ancestor was living at each point of divergence; for example, there was a common ancestor for monkeys, apes, and humans about 33 mya; one for all apes and humans about 15 mya; and one for just African apes and humans about 7 mya.
Figure 19.37Human evolution.
The length of time each species existed is indicated by the vertical lines. Notice that there have been times when two or more hominids existed at the same time, such as the three types of australopithecines shown. Therefore, human evolution resembles a “bush” rather than a single branch of a phylogenetic tree.
Check Your Progress
Contrast the australopithecines with Homo habilis.
Answer:Australopithecines were apelike above the waist and humanlike below the waist. Homo habilis had a larger brain and smaller cheek teeth than the australopithecines, and was able to use stone tools and possibly speech. 
Figure 19.39Evolution of modern humans.
a. The multiregional continuity hypothesis proposes that Homo sapiens evolved separately in at least three different places: Asia, Africa, and Europe. Therefore, continuity of genotypes and phenotypes is expected in each region, but not between regions. b. The out-of-Africa hypothesis proposes that Homo sapiens evolved only in Africa; then this species migrated and supplanted populations of Homo in Asia and Europe about 100,000 years ago.
Figure 19.38Australopithecus afarensis.
a. A reconstruction of Lucy on display at the St. Louis Zoo. 
b. These fossilized footprints occur in ash from a volcanic eruption some 3.7 mya. The larger footprints are double, and a third, smaller individual was walking to the side. (A female holding the hand of a youngster may have been walking in the footprints of a male.) The footprints suggest that A. afarensis walked bipedally.
Check Your Progress
	1.	What are some distinctive features of Homo erectus compared to Homo habilis?
	2.	What is the out-of-Africa hypothesis?
	3.	Who were the Neandertals?
Answers:1. Homo erectus had a larger brain and flatter face, most likely a striding gait as we do, used fire, and made better tools.2. The out-of-Africa hypothesis proposes that Homo sapiens evolved only in Africa and then migrated to other parts of the world.3. The Neandertals were a stocky and powerfully muscular people capable of surviving harsh Ice Age conditions. They are believed to have been culturally advanced.
Figure 19.40Cro-Magnons.   
Cro-Magnon people are the first to be designated Homo sapiens. Their tool-making ability and other cultural attributes, including their artistic talents, are legendary.
Bipedal
Tool Use
Brain Size Increased
d.
c. Sea cucumber
b. Sea lily (above), feather star (right)
d. Brittle star
e. Sea urchin (left), sand dollar (right)
upstroke
downstroke
Stages in development, from zygote to embryo.
Stages in metamorphosis, from hatching to tadpole.
Adult frog
Figure 19.1Animals.
Animals begin life as a fertilized egg. The egg undergoes development to produce a multicellular organism that has specialized tissues. Animals depend on a source of external food to carry on life’s processes. This series of images shows the development and metamorphosis of the frog, a complex animal. 
Housefly
Tortoiseshell scale
Flea
Dragonfly
Honeybee
Luna moth
Lubber grasshopper
True bug
Check Your Progress
	1.	Describe how reptiles are adapted to reproduce on land.
	2.	
List the anatomical features of birds that help them fly.
Answers:1. Fertilization is internal in reptiles and results in the production of a leathery amniote egg.2. Birds’ forelimbs are modified as wings; their bones are usually hollow with air cavities; air sacs involved in respiration also lighten the body; and they have a well-developed nervous system and muscles.
Green lacewing
Snout beetle
stone tools