Concept 23.1   Distinct Body Plans Evolved among the Animals

• Animals share a set of derived traits not found in other groups of organisms, including similarities in the sequences of many of their genes, the structure of their cell junctions, and the components of their extracellular matrix. Review Figure 23.1
• Patterns of embryonic development provide clues to the evolutionary relationships among animals. Diploblastic animals develop two embryonic cell layers; triploblastic animals develop three cell layers.
• Differences in their patterns of early development characterize two major clades of triploblastic animals, the protostomes and the deuterostomes.
• Animal body plans can be described in terms of symmetry, body cavity structure, segmentation, and type of appendages.
• Most animals have either radial symmetry or bilateral symmetry. Many bilaterally symmetrical animals exhibit cephalization, with sensory and nervous tissues in an anterior head.
• On the basis of their body cavity structure, animals can be described as acoelomates, pseudocoelomates, or coelomates. Review Figure 23.4 and ACTIVITY 23.1
• Segmentation takes many forms and improves control of movement, especially if the animal also has appendages.

Concept 23.2   Some Animal Groups Fall outside the Bilateria

• All animals other than sponges, ctenophores, placozoans, and cnidarians belong to a large, monophyletic group called the Bilateria. Eumetazoans, which have tissues organized into distinct organs, include all animals other than sponges. Review ACTIVITY 23.2
• Sponges are simple, asymmetrical animals that lack differentiated cell layers and true organs. Sponges have skeletons made up of silicaceous or calcareous spicules. They create water currents and capture food with flagellated feeding cells called choanocytes. Choanocytes are an evolutionary link between the animals and the choanoflagellate protists.
• The two cell layers of the radially symmetrical ctenophores are separated by an inert extracellular matrix called mesoglea. Ctenophores move by beating comblike plates of cilia called ctenes. Review Figure 23.7
• The life cycle of most cnidarians has two distinct stages: a sessile polyp stage and a motile medusa. A fertilized egg develops into a free-swimming larval planula, which settles to the bottom and develops into a polyp. Review Figure 23.9 and ANIMATED TUTORIAL 23.1

Concept 23.3   Protostomes Have an Anterior Brain and a Ventral Nervous System

• Protostomes (“mouth first”) are bilaterally symmetrical animals that have an anterior brain surrounding the entrance to the digestive tract and a ventral nervous system. Protostomes comprise two major clades, the lophotrochozoans and the ecdysozoans. Review ANIMATED TUTORIAL 23.2
• Lophotrochozoans include a wide diversity of animals. Within this group evolved lophophores (a complex organ for food collection and gas exchange) and free-living trochophore larvae. These features were subsequently lost in some lineages. The exact placement of arrow worms within the protostomes remains uncertain.
• Lophophores, wormlike body forms, and external shells are each found in many distantly related groups of lophotrochozoans. The most species-rich groups of lophotrochozoans are the flatworms, annelids, and mollusks.
• Annelids are a diverse group of segmented worms that live in moist terrestrial and aquatic environments. Review Figure 23.17 and Figure 23.18
• Mollusks underwent a dramatic evolutionary radiation based on a body plan consisting of three major components: a foot, a mantle, and a visceral mass. The four major living molluscan clades—chitons, bivalves, gastropods, and cephalopods—demonstrate the diversity that evolved from this three-part body plan. Review Figure 23.19
• Ecdysozoans have a cuticle covering their body, which they must molt in order to grow. Some ecdysozoans, notably the arthropods, have a rigid cuticle reinforced with chitin that functions as an exoskeleton. New mechanisms of locomotion and gas exchange evolved among the arthropods.
• Nematodes, or roundworms, have a thick, multilayered cuticle. Nematodes are among the most abundant and universally distributed of all animal groups. Review Figure 23.22
• Horsehair worms are extremely thin; their larvae are internal parasites.
• Many ecdysozoan groups are wormlike in form. Members of several species-poor groups of marine ecdysozoans—priapulids, kinorhynchs, and loriciferans—have thin cuticles.
• One major ecdysozoan clade, the arthropods, has evolved jointed, paired appendages that have a wide diversity of functions.

Concept 23.4   Arthropods Are Diverse and Abundant Animals

• Arthropods are the dominant animals on Earth in number of described species, and among the most abundant in number of individuals.
• Encasement within a rigid exoskeleton provides arthropods with support for walking as well as some protection from predators. The waterproofing provided by chitin keeps arthropods from dehydrating in dry air.
• Jointed appendages permit complex movements. Each arthropod segment has muscles attached to the inside of the exoskeleton that operate that segment and the appendages attached to it.
• Two groups of arthropod relatives, the velvet worms and the tardigrades, have simple, unjointed appendages.
• Chelicerates have a two-part body and pointed mouthparts that grasp prey; most chelicerates have four pairs of walking legs.
• Mandibles and antennae are synapomorphies of the mandibulates, which include myriapods, crustaceans, and hexapods.
• Crustaceans are the dominant marine arthropods and are also found in many freshwater and some terrestrial environments. Their segmented bodies are divided into three regions (head, thorax, and abdomen) with different, specialized appendages in each region.
• Hexapods—insects and their relatives—are the dominant terrestrial arthropods. They have the same three body regions as crustaceans, but no appendages form in their abdominal segments. Wings and the ability to fly first evolved among the insects, allowing them to exploit new lifestyles. Review Figure 23.30

Concept 23.5   Deuterostomes Include Echinoderms, Hemichordates, and Chordates

• Deuterostomes vary greatly in adult form, but based on the distinctive patterns of early development they share and on phylogenetic analyses of gene sequences, they represent a monophyletic group. There are far fewer species of deuterostomes than of protostomes, but many deuterostomes are large and ecologically important. Review ANIMATED TUTORIAL 23.3
• Echinoderms and hemichordates both have bilaterally symmetrical, ciliated larvae.
• Most adult echinoderms have pentaradial symmetry. Echinoderms have an internal skeleton of calcified plates and a unique water vascular system connected to extensions called tube feet. Review Figure 23.32
• Hemichordate adults are bilaterally symmetrical and have a three-part body that is divided into a proboscis, collar, and trunk. They include the acorn worms and the pterobranchs. Review Figure 23.34
• Chordates fall into three principal subgroups: lancelets, tunicates, and vertebrates.
• At some stage in their development, all chordates have a dorsal hollow nerve cord, a postanal tail, and a notochord. Review Figure 23.35
• Tunicates include the sea squirts, which are sessile filter feeders as adults. Lancelets live buried in the sand of shallow marine and brackish waters.
• The vertebrate body is characterized by a rigid internal skeleton, which is supported by a vertebral column that replaces the notochord, internal organs suspended in a coelom, a ventral heart, and an anterior skull with a large brain. Review Figure 23.37
• The evolution of jaws from gill arches enabled individuals to grasp large prey and, together with teeth, cut them into small pieces. Review Figure 23.40
• Chondrichthyans have skeletons of cartilage; almost all species are marine. The skeletons of bony vertebrates are made of bone. Among the bony vertebrates, ray-finned fishes have colonized most aquatic environments.

Concept 23.6   Life on Land Contributed to Vertebrate Diversification

• Lungs and jointed appendages enabled one lineage of lobe-limbed vertebrates to colonize the land. This lineage gave rise to the tetrapods. Review Figure 23.44
• The earliest split in the tetrapod tree is between the amphibians and the amniotes (reptiles and mammals).
• Most modern amphibians are confined to moist environments because they and their eggs lose water rapidly. Review Figure 23.45 and ANIMATED TUTORIAL 23.4
• An impermeable skin, efficient kidneys, and an egg that could resist desiccation evolved in the amniotes. Review Figure 23.47 and ACTIVITY 23.3
• The major living reptile groups are the lepidosaurs (tuataras, lizards, snakes, and amphisbaenas), turtles, and archosaurs (crocodilians and birds). Review Figure 23.48
• Mammals are unique among animals in supplying their young with a nutritive fluid (milk) secreted by mammary glands. There are two primary mammalian clades: the prototherians (of which there are only five species) and the species-rich therians. The therian clade is subdivided into the marsupials and the eutherians. Review Table 23.3 Part 1 and Table 23.3 Part 2

Concept 23.7   Humans Evolved among the Primates

• Grasping limbs with opposable digits distinguish primates from other mammals. The prosimian clade includes the lemurs, lorises, and galagos; the anthropoid clade includes tarsiers, monkeys, apes, and humans. Review Figure 23.54
• Hominid ancestors developed efficient bipedal locomotion. Several species of Homo coexisted in parts of the world until recently. Review Figure 23.55
• In the lineage leading to Homo, brains became larger as jaws became smaller. The two events appear to be developmentally linked and are an example of evolution via neoteny. Review Figure 23.56

See ACTIVITY 23.4 for a concept review of the major groups of organisms.