An Introduction to Zoology: Chapter 11

The members of the clade Ecdysozoa are an even more diverse group than are the Lophotrochozoa. Like the lophotrochozoans, however, the ecdysozoans appear to be a genuine clade. What distinguishes the ecdysozoans is that they have an outer, non-living cuticle that functions as an external skeleton (an exoskeleton).

The cuticle is secreted by an ecdysozoan’s epidermis (skin), and covers the body, providing a more or less rigid structure that provides protection against injury and, in many species, against dehydration. In many species, the cuticle also serves as an attachment-site for muscles, allowing for more efficient movement.

One disadvantage of a cuticle is that since it is not made of living tissue, it does not grow with the animal. This means that it must be shed (molted) on occasion, to permit its owner’s growth.


The ecdysozoans are a much larger and more diverse group of animals than are the lophotrochozoans. The lophotrochozoans, in addition to being a smaller group, tend to have rather simpler body plans. So, we’ll begin our discussion of bilaterian diversity with the lophotrochozoans. Specifically, we’ll begin with the simplest of the lophotrochozoans, the acoelomates.