An Introduction to Zoology: Chapter 11

The flatworms, mesozoans and ribbon worms are generally considered to be the simplest of the Bilateria. That doesn’t mean they aren’t successful, however; the platyhelminths, in particular, are a large and diverse group of animals.

There are some lophotrochozoan phyla that are much larger and more diverse than are the platyhelminths, however. In particular, the phyla Mollusca and Annelida are very large and diverse animal taxa, and everyone is familiar with representatives of these phyla. We’ll discuss those phyla shortly, but first we’ll discuss some of the smaller and less-familiar lophotrochozoan phyla.