Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Metazoa
Annelids

General characteristics

There are approximately 9,000 known species of segmented worms, or Annelids. They inhabit marine and freshwater environments, as well as soil. Compared to Flatworms and Roundworms, annelids occupy a higher evolutionary grade and are crucial for understanding the evolution of higher invertebrates.

The Main characteristics of the phylum are as follows: the body always consists of a greater or lesser number of rings, or segments (segmentation is evident not only externally, but also internally, with the repetition of many Internal Organs); There is a secondary body cavity (coelom) lined with epithelial Cells that envelop organs and Tissues, separating them from the coelomic fluid. The coelomic cavity of annelids is also partitioned by septa that correspond, to a greater or lesser extent, to the external segmentation; a well-developed closed Circulatory system is present; and excretory organs are of the open type, known as nephridia or metanephridia. These are delicate, paired tubules found in nearly every segment, beginning with a ciliated funnel in the body cavity. The Nervous system consists of a paired supra-esophageal ganglion, referred to as the Brain, which is connected by nerve cords to a ventral pair of nerve trunks forming the ventral nerve cord.

The phylum of segmented worms (Annelida) comprises several classes, of which the most widely distributed and well-known are Polychaetes (Polychaeta), Oligochaetes (Oligochaeta), and Leeches (Hirudinea).