The coelomates can be divided into two large groups that reflect their evolutionary history. These are the protostomes and the deuterostomes. Differences between the two groups are apparent in their embryonic development. The word “protostome” has roots meaning “mouth first.”

Let’s consider the development of the digestive tract of a protostome. The infolding of cells forming the pouch of the gastrula is the structure that eventually becomes the digestive cavity. The first opening here will ultimately become the mouth of the organism—thus, “mouth first.” Later, a second opening to the cavity will become the organism’s anus, where digestive wastes are expelled.

The roots of the word “deuterostome” mean “mouth second.” In deuterostome development, the first opening to the primitive digestive cavity will eventually become the animal’s anus. The second opening will ultimately become the animal’s mouth—thus, “mouth second.”

Let’s conclude our introduction to animals with a look at one of the most important phyla of protostomes: the segmented worms called annelids. The annelids include the familiar earthworms, which ingest soil and extract its nutrients. Leeches are also annelids.

Some species are well known as bloodsucking parasites of humans and other animals. Many leeches prey on other small animals. Still other annelids are marine worms, which bear distinctive protrusions and bristles.

Many annelids are hermaphrodites: they possess both male and female reproductive organs. When two hermaphrodites mate, they exchange sperm and fertilize each other’s eggs.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education