Askjagden's Guide to Biology: Annelids
Phylum Annelida contains members called annelids. If you have studied Latin, you would know that the word anulus means ring. Annelids are named after that word, because they are segmented worms. They are the only segmented worms; flatworms and roundworms are not segmented. These segments are called septa. Annelids move through bristles called setae. They are the first to have genuine coeloms (a coelom is a body cavity made up of mesoderm). Annelids are invertebrates.
Annelids are actually quite complex. Annelids feed with forms that are quite helpful; for example, some annelids have jaws. Some are filter feeders. In some more complex annelids, the food moves into the pharynx then down the esophagus to two digestive organs: first the crop, which stores the food, and then the gizzard, which breaks down the food. They have closed circulatory systems (which means that they have circulatory systems with blood enclosed inside a system of a heart and blood vessels). Most annelids usually breathe with gills. Cellular waste is nitrogen; it is excreted through forms called nephridia. Digestive waste is excreted through the anus. Annelids have a well developed nervous system: they have brains and many nerves. They usually move with setae, but they can also move through the use of muscles to push themselves forward. Annelids usually reproduce sexually through internal fertilization. Most are hermaphrodites. The earthworm is a type of annelid that has a large, center band that is used for fertilization. That band is called a clitellum. Earthworms will use them for fertilization.
The phylum Annelida is divided into three classes. The first is Oligochaeta. Its members are called oligochaetes. The class contains earthworms and their relatives. They have a body shape that facilitates movement and do not have as much setae as polychaetes, another group of annelids. Although earthworms usually live in soil or fresh water, evidence of earthworms on ground can be shown through castings, which are squiggles of mud left by them.
The second class is called Hirudinea; its members are known as leeches. Leeches are external parasites that suck blood and fluids from other animals. Leeches have powerful forms that they use to do so. For example, suckers can anchor them to other organisms. Jaws or proboscises help slice into the organism.
The final class is Polychaeta, which contains the polychaetes. It contains other worms beside earthworms. They have appendages tipped with a lot of setae. "Poly-" means many; "oligo-" means few. Chaeta refers to bristles or seta; therefore polychaetes refers to a lot of seta, whereby oligochaetes refer to too few seta,
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