a. Most animals are built on a fundamental plan involving a series of segments called metameres or somites.

b. Each metamere is a kind of "unit animal" containing the most essential body parts (Figure 35.1).

c. Not all apparently segmented animals have true functional segmentation.

d. Research on Hox genes has shed new light on the importance of metamerism; groups like the chordates, the annelids, and the arthropods share Hox genes, implying the fundamental plan for animal development was established in a common ancestor.

e. Metamerism apparently evolved early in the protostome clade, and dominates the structures of annelids and arthropods.

f. We can understand the structure and evolution of the segmented animals by following the development of their metamerism.

g. A segmented coelomic cavity has advantages for movement (Figure 35.2).

a. Worms of the phylum Annelida have a classical coelomate body plan, with a tubular gut running through the middle of a coelom (Figure 35.3).

b. Annelids have a closed circulatory system with a dorsal aorta and a series of hearts.

c. Annelids have a pair of metanephridia in each segment, a ventral nerve cord, and a simple brain made of a concentration of nerve tissue in the head end.

d. Both specialized sensory cells and chemoreceptors allow the annelids to discriminate among various stimuli.

e. Earthworms belong to the class Oligochaeta (few bristles).

f. The class Polychaeta (many bristles) contains the mostly marine annelids that use the metameric organization of their muscles and their short, flat parapodia to facilitate rapid movement along the ocean floor (Figure 35.4).

g. A polychaete contracts the muscles on each side of its body in a wave running from the tail to the head, as shown in Figure 35.5.

h. Class Hirudinea contains the leeches, which move by attaching the head to the substrate, bringing up the tail, releasing the head end and pushing it forward again (Figure 35.6).

a. Peripatus (Figure 35.7), of phylum Onychophora, is a tropical forest creature that looks like a highly modified annelid but also has arthropod features; it thus represents a possible link between these two groups.

b. In discussing the animal evolution, features shared with earlier groups (e.g. the closed circulatory system of annelids) are shared primitive characteristics; and features shared with more recently evolved groups (e.g. the open circulatory system of arthropods) are shared derived characteristics.

c. Peripatus is metameric, with a gut and nervous system like those of an annelid, and with coxal glands for excretion in each segment.

d. Each Peripatus appendage ends in a pair of claws, it has an open circulatory system and hemolymph that serves as both blood and lymph, and a dorsal heart, like an arthropod.

a. Arthropods constitute the largest of the animal phyla, are metameric, and have specialized segments and specialized, segmented limbs.

b. Most arthropods have some limbs for locomotion and other limbs specialized for grasping, swimming, or eating (Figure 35.8).

c. Arthropods are classified in part on the basis of whether their limbs are uniramous or biramous.

d. Arthropods have a tough exoskeleton composed of chitin and hardened in to a rigid cuticle that is covered with waxy lipids and is waterproof.

e. The exoskeleton serves as an attachment site for muscles, and also protects the animal from predation and dessication.

f. One disadvantage of the exoskeleton is that it must be shed periodically in order for the animal to grow or repair injuries, so arthropods molt in a process called ecdysis.

g. Most arthropods have a movement sensitive compound eye composed of many units called ommatidia; each of which has a lens and light-sensitive cell (Figure 35.9).

h. Arthropods have an open circulatory system (Figure 35.10) with one fluid, the hemolymph, traveling through arteries into sinuses, from which all tissue cells receive their nutrients and into which they deposit wastes.

i. The arthropod hemolymph travels through an open space called a hemocoel.

j. While aquatic arthropods have gills, most air-breathing arthropods have a system of air tubes called tracheae to carry air directly to their tissues.

k. Some spiders and other land arthropods exchange gases with a book lung (Figure 35.11).

l. Air-breathing arthropods have small body openings called spiracles, through which the air passes into the tracheae or lungs.

m. Arthropods are divided into four subphyla: Trilobitomorpha, Cheliceriformes, Uniramia, and Crustacea, based on the specialization of their appendages.

n. The trilobites, all now extinct, had bodies divided into a central lobe and two lateral lobes, with relatively uniform, unspecialized legs (Figure 35.12).

o. The chelicerates include mostly the arachnids–spiders, scorpions, and their relatives (Figure 35.13)–which have appendages modified into pedipalps and chelicerae.

p. The chelicerate body is divided into an anterior cephalothorax bearing mouthparts and four pairs of legs, and a posterior abdomen, which does not bear appendages except for sexual or respiratory structures.

q. Crustaceans (Figure 35.14) are characterized by a long trunk and a head made of five fused segments bearing specialized appendages:

r. Uniramians are divided into two classes, all of which bear antennae, mandibles, and two pairs of maxillae on their heads:

s. Many uniramians (Figure 35.15) are reminiscent of annelids, with their uniform segmentation.

t. The insects have improved and simplified the basic walking method evolved in annelids and still used in spiders, centipedes, and millipedes, using extensor muscles rather than hydrostatic skeletons to extend their legs independently.

u. Figure 35.16 compares the walking motions of a millipede and an insect.

v. Figure 35.17 summarizes the most important orders of insects.

a. The molluscs are a highly varied group that include clams, oysters, snails, octopuses, chitons, and squid.

b. Though many molluscs secrete a shell, the name of the phylum Mollusca means soft-bodied.

c. The basic molluscan body plan is shown by a chiton (class Polyplacophora or Amphineura), a bilaterally symmetrical animal covered with eight separate plates or valves (Figure 35.18).

d. The typical molluscan body is divided dorsoventrally into three parts:

e. Molluscs are bilaterally symmetric, as is obvious in the class Bivalvia, where each half of the shell is a valve.

f. The molluscan body is highly modified in the univalves, which have a single valve with only one opening that is used for both the mouth and anus, and whose bodies have thus been twisted.

g. Figure 35.19 shows the twisted body plans of univalve molluscs in the class Gastropoda, which includes the snails, whelks, cowries, and other animals that construct spiral shells.

h. The body is also twisted in the tooth shells (Scaphopoda) and in the chambered nautilus of the class Cephalopoda.

i. Molluscs have an open circulatory system, a heart that pumps hemolymph through arteries to open sinuses (Figure 35.20), gills for respiration, and nephridia for excretion.

j. The mollusc digestive tract includes a large stomach and digestive gland (liver).

k. Some molluscs (e.g. gastropods and chitons) feed on large, solid food by rasping it with a specialized radula, which cuts and scrapes food into small fragments that are fed into an esophagus.

l. Bivalves feed by pumping water through the chamber around their gills and filtering out suspended food particles.

m. Molluscs have concentrations of ganglia at the head end, connected by a ring of nerves running around the esophagus and a pair of ventral nerve cords.

n. The muscular mollusc foot in most species allows the animals to move across surfaces, or burrow through mud (Figure 35.21).

o. Some molluscs (e.g. the nudibranchs) have the foot modified into winglike swimming appendages (Figure 35.22).

p. Cephalopods move rapidly by expelling a stream of water with a contraction of their muscular mantles.

q. In 1957, Henning Lemche reported on a newly found mollusc, Neopilina galatheae (Figure 35.23), which has five segments that each contain paired gills and paired nephridia, and was thought to be a link between the metameric annelids and arthropods.

r. Further examination has shown that Neoplina has irregular metamerism, and may not be a link, after all.

s. Molluscs probably arose from a protostome clade sometime before the annelid-arthropod line evolved.

a. Animals in the phyla Phoronidea, Brachiopoda, and Bryozoa, all have U-shaped intestines, are sessile marine animals, and feed with a lophophore consisting of a pair of spiral ridges fringed with many ciliated tentacles that sweep water toward the mouth.

b. The phoronids consist of 15 species of wormlike animals (Figure 35.24) that lie in the ocean bottom with their lophophores exposed to collect food.

c. The brachiopods (Figure 35.24) look very much like clams, but with dorsal and ventral halves, and their lophophores occupy much of the space inside their shells.

d. The ectoprocts, or bryozoa, are small "moss animals" that grow in colonies;each animal looks like a phoronid inside a case (Figure 35.24).

a. Most members of the phylum Echinodermata, which includes sea stars, sea urchins, sand dollars, and their relatives, move about freely but have radial symmetry (Figure 34.25).

b. Though modern echinoderms move freely, ancestral echinoderms were adapted to a stationary life, and the sessile echinoderms comprise a subphylum that is mostly extinct except for a few crinoids.

c. Throughout this phylum, fivefold symmetry is common, but sea stars have many arms that do not all occur in multiples of five.

d. The sea cucumber, class Holothuroidea, have evolved more of a cephalized, bilaterally symmetrical shape.

e. A sea star (Figure 35.26) illustrates the general echinoderm body plan:

a. The chordates include most of the other deuterostome phyla, and include the vertebrates.

b. All chordates share three features that are present at some stage in their life cycles (Figure 35.27):

c. The chordates are divided into three subphyla: the Urochordata, the Cephalochordata, and the Vertebrata.

d. The urochordates include the tunicates, which resemble leathery bags and are pelagic marine animals that are also called sea squirts (Figure 35.28).

e. The sea squirt body has a large pharynx with gill slits, and cilia that move water through the animal as it filter feeds and exchanges gases with its environment.

f. Sea squirt larvae (Figure 35.28) show all the chordate features, include a notochord in its tail, for which the subphylum was named.

g. The subphylum Cephalochordata contains the amphioxus or lancelet, which looks like a small fish and has all three chordate features as an adult (Figure 35.29).

h. Amphioxus shows clear metamerism; it is divided lengthwise into a series of muscle segments and its embryo has many gill slits with a protonephridium associated with each one.

i. Vertebrates retain the metamerism of the amphioxus in their internal structures.

a. Members of the subphylum Vertebrata are distinguished by an endoskeleton of bone or cartilage, centering around a vertebral column and made of a series of separate vertebrae.

b. The internal skeleton combines flexibility with enough strength to support a large body.

c. Other vertebrate features include:

a. As vertebrates, humans have a special interest in the group, shown in Figure 35.30.

b. The earliest vertebrates were small, jawless fishes (agnathans) that appeared during the Ordovician period, 500 million years ago, and lived as bottom dwellers that fed on suspended food particles.

c. The agnathans were also called the ostracoderms because their bodies were covered with bony plates, and they had a head shield covering their brain (Figure 35.31).

d. The only living agnathans are cyclostomes, also known as lampreys and hagfishes (Figure 35.31), which lack the paired fins of advanced fishes and have a persistent notochord that is never replaced by backbone.

e. Table 35.1 shows the classification of various types of fish.

f. The ostracoderms gave rise to the first jawed fishes around 400 million years ago.

g. The evolution of jaws around the mouth was one of the most critical in all of vertebrate evolution, opening the way for an enormous variety of feeding mechanisms and thus a large range of adaptive zones.

h. The story of jaw evolution from the cartilage of some gill arches is told in Sidebar 44.1.

i. The acanthodians retained the bony plates of the ostracoderms and were probably ancestral to modern scaly fishes.

j. Cartilaginous fishes are sometimes thought to be more primitive than bony fishes, but sharks and rays apparently evolved from ancestors with bony skeletons.

k. Bony fishes have a variety of external forms, all based on the same basic anatomy–a highly metamerized series of vertebrae, ribs, and associated muscle segments (Figure 35.32).

l. Most bony fishes have a ray-fin structure, but the lobe-fin fishes (Crossopterygii) have fins with a fleshy lobe.

m. One group of Devonian lobe-fins (rhipidistians) had lungs and internal nostrils and were the ancestors of amphibians.

n. The lobe-fins are represented today only by the lungfishes and a deep-sea fish, Latimeria, discovered off the east coast of Africa and a living member of the coelacanths, a group previously thought to be extinct.

a. Living amphibians include newts, salamanders, frogs, and toads, and legless creatures of the tropics called blindworms or caecilians.

b. Amphibians are the remnants of a once larger group that flourished from about the Mississippian to the Triassic periods, about 350 to 200 million years ago.

c. Amphibians were the first real pioneers of land life, becoming adapted to a life of breathing air while not always surrounded by water.

d. The amphibian life cycle begins with eggs laid in the water, which develop into aquatic larvae with external gills that are resorbed as the larvae develops lungs and limbs and become adults.

e. Many salamanders grow directly from embryos into the adult stage without passing through the larval stage; adult salamanders are sometimes confined to an aquatic life of moist places.

f. Axolotls are salamanders that never achieve morphological adulthood and remain swimming larvae with gills and sexual maturity (a phenomenon called neotony) all their lives (Figure 35.33).

g. The labyrinthodonts (labyrinthine teeth) are the first known amphibians, descending directly from the crossopterygians during the Devonian period.

h. Early amphibians evolved lungs, strong limbs for movement on land, and tougher, less permeable skin to prevent water loss during a terrestrial existence.

i. The development of the pelvic and pectoral girdles from the weak girdles that anchor fish fin bones was a major step in land vertebrate evolution (Figure 35.34).

j. Primitive reptiles apparently evolved from amphibians soon after the latter had arisen, and the two groups were in competition from the beginning.

a. Amphibians had terrestrial niches to themselves from about the Devonian to the Permian period, before reptiles came to dominate the earth for about 200 million years.

b. The birds and mammals that came later are merely slightly altered reptiles, and Reptilia is a paraphyletic taxon if it is not defined to include the birds and mammals.

c. Modern reptiles include snakes, lizards, turtles, crocodiles, and Sphenodon, of New Zealand (Figure 35.35).

d. Modern reptiles represent the remnants of a group of often huge creatures that lasted until about 70 million years ago.

e. There are fundamental differences between the anatomy of amphibians and reptiles:

f. The first reptilian animals appeared in the late Pennsylvanian period, about 300 million years ago.

g. Reptile subclasses are defined by the presence or absence of extra openings in the side of the skull (Figure 35.36).

h. The first reptiles, anapsids, had solid skulls as do the turtles of today.

i. Other reptiles developed along several lines (Figure 35.37) during the next 150 million years.

j. The extinction of the pterosaurs and dinosaurs is still a mystery.

a. In the mid-1970s, Robert Bakker and John Ostrom examined dinosaur bone structures and presented evidence that suggests that dinosaurs were endothermic, like their bird and mammalian descendants.

b. Ostrom also examined small dinosaurs and realized they were very similar to Archaeopteryx (Figure 35.39), the first animal classified as a bird.

c. Bakker and Ostrom have proposed that birds evolved from dinosaurs, which were already endothermic (Figure 35.38), and suggested that feathers evolved first for their insulating value, rather than for flight.

d. The demands of being light enough to fly surely selected the major characteristics of birds (Figure 35.40).

e. Birds have an efficient cardiovascular system to supply the oxygen needed for sustained flight.

f. In birds, air passes in one direction through the lungs (Figure 35.41), increasing the efficiency of blood oxygenation.

g. The class Aves is divided into numerous orders and families, but most birds are perching birds of the order Passeriformes, which include the sparrows, larks, thrushes, warblers, and other small birds.

h. The various orders of birds are distinguished by specializations of their feet and bills.

a. Mammals are derived from therapsid reptiles of the Triassic period.

b. In mammals, the reptilian scales have been modified into hair, and females have milk glands, or mammae, for suckling their young.

c. Strict definitions of mammalian classes revolve around the tiny bones in the middle ear.

d. Mammals have specialized teeth, incisors for cutting and molars for grinding, and a distinctive ball-joint at the base of the skull.

e. Table 35.2 presents the classification of mammals.

f. Two subclasses and three infraclasses are separated mainly on the basis of their reproductive modes.

g. Monotremes (the duck-billed platypus and the spiny anteater) lay leathery-shelled eggs like those of turtles (Figure 35.42).

h. Marsupials (kangaroos, wombats, and opossums) have embryos that leave the uterus partway through development, and move to a marsupium (pouch) on the mother to finish growing (Figure 35.43).

i. The placental mammals (Figure 35.44) have a placenta which attaches to the uterus containing the developing embryo, and whose blood supply exchanges gases, supplies nutrients, and removes wastes.