recap

31.1 recap

The shared derived traits of protostomes include a blastopore that develops into a mouth (except in arrow worms), an anterior brain, and a ventral nervous system. Several lophotrochozoan groups are characterized by a filter-feeding ciliated structure known as a lophophore or by cilia-bearing larvae known as trochophores. Ecdysozoans, which have a body covering known as a cuticle, must molt periodically in order to grow.

learning outcomes

You should be able to:

  • Synthesize the consequences of having a hard body covering versus a soft body for life as an animal.

  • Describe major evolutionary innovations among protostomes that are related to life in aquatic versus terrestrial environments.

Question 1

How does an animal’s body covering influence the way it exchanges gases, feeds, and moves?

A soft body covering maximizes flexibility and body conformation, allows movement into confined spaces, and permits direct exchange of gases and fluids with the environment. Movement of soft-bodied organisms is largely through the actions of muscles acting on a hydrostatic skeleton. A hard body covering, in contrast, provides greater protection from predation and dehydration and provides a structure to anchor muscles that attach to appendages, which can assist in movement and feeding. Respiration in a hard-bodied organism must be accomplished through external openings and movement of gases inside the body.

Question 2

What features make arthropods well adapted for colonizing terrestrial environments?

Their jointed appendages and rigid exoskeletons provide support for walking on land. The exoskeletons also provide protection against dehydration, which is important in a terrestrial environment.

In the next section we continue our survey of the protostomes with a more detailed look at the major groups of lophotrochozoans and the diverse body forms that are found among them.