Chapter Summary

• Protostomes (“mouth first”) are bilaterally symmetrical animals with an anterior brain that surrounds the entrance to the digestive tract and a ventral nervous system. The embryonic blastopore of protostomes develops into a mouth.

• There are two major clades of protostomes, the lophotrochozoans and the ecdysozoans. Review Figure 31.1, Table 31.1, Activities 31.1, 31.2

• Lophotrochozoans include a wide variety of body forms. Within this group, lophophores (complex organs for both food collection and gas exchange), free-living trochophore larvae, and spiral cleavage evolved. Some of these features were subsequently lost in some lineages (or evolved convergently).

• Ecdysozoans have a body covering known as the cuticle, which they must molt in order to grow. Some ecdysozoans, especially the arthropods, have a rigid cuticle reinforced with chitin that functions as an exoskeleton. Review Figure 31.4

• Arrow worms may be most closely related to lophotrochozoans, or they may be the sister group of all other protostomes. Review Figure 31.5

• Lophotrochozoans range from animals with a blind gut and no internal transport system to animals with complete digestive tracts and complex internal transport systems. Review Figure 31.6

• Most species of bryozoans and entoprocts live in colonies produced through asexual reproduction. Individuals of both groups feed using a lophophore.

• Flatworms, rotifers, gastrotrichs, and their close relatives form a structurally diverse clade of ciliated lophotrochozoans. Review Figure 31.7

• Ribbon worms feed using a long, protrusible proboscis. Review Figure 31.8

• The shelled brachiopods and wormlike phoronids use a lophophore to feed; this lophophore may have evolved independently of the lophophore in bryozoans and entoprocts. Review Figures 31.9, 31.10

• Annelids are a diverse group of segmented worms that live in moist terrestrial and aquatic environments. Review Figure 31.11

• 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 31.13

• Several species-poor groups of wormlike marine ecdysozoans—priapulids, kinorhynchs, and loriciferans—have thin cuticles.

• Nematodes have a thick, multilayered cuticle. Nematodes are among the most abundant and universally distributed of all animal groups. Review Figure 31.16

• Horsehair worms are extremely thin; many are endoparasites as larvae.

• One major ecdysozoan clade, the arthropods, has evolved jointed, paired appendages that have a wide diversity of functions. Collectively, arthropods are the dominant animals on Earth in number of described species.

• 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 movement patterns. Each arthropod segment has muscles attached to the inside of the exoskeleton that operate that segment and attached appendages.

• The onychophorans and the tardigrades are arthropod relatives that have simple, unjointed appendages. Trilobites, the first arthropods known to have had jointed appendages, disappeared in the Permian mass extinction.

• 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 the myriapods, crustaceans, and hexapods.

• The bodies of myriapods have two regions: a head with mandibles and antennae, and a segmented trunk that bears many pairs of legs.

• Crustaceans have segmented bodies that are divided into three regions—head, thorax, and abdomen—with different, specialized appendages in each region. Review Figure 31.24

• 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. Review Figure 31.26, Table 31.2

• Wings and the ability to fly first evolved among the pterygote insects, allowing them to exploit new lifestyles. Review Figure 31.28