31.1 Protostomes Account for More Than Half of All Described Species
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-
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
31.2 Many Lophotrochozoans Have Ciliated Feeding Structures or Life Stages
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—
31.3 Ecdysozoans Grow by Shedding Their Cuticles
Several species-
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.
31.4 Arthropods Are the Most Abundant and Diverse Group of Animals
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-
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—
Hexapods—insects and their relatives—
Wings and the ability to fly first evolved among the pterygote insects, allowing them to exploit new lifestyles. Review Figure 31.28
See Animation 31.1 for an overview of the protostomes.
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