Practical Guide to Zoology: Textbook - T. A. Dauda 2014

Multicellular Animals
Annelids

General characteristics

There are approximately 9,000 known species of Annelids, or segmented worms. 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.

Annelids are the most highly evolved worms.

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 expressed not only in external Structure, but also internally, with the repetition of many Internal Organs);

✵ There is a secondary body cavity (coelom) lined with epithelial Cells that enclose organs and Tissues, separating them from the coelomic fluid. The body cavity of annelids is also divided by septa, corresponding more or less to the external segmentation;

✵ many annelids possess rudimentary limbs in the form of paired, muscular outgrowths on the segments—parapodia—equipped with bundles of setae;

✵ a Skin-Muscle tube surrounds the secondary body cavity, the coelom, which is lined with coelomic epithelium (endothelium);

✵ the digestive tract is tubular and differentiated into foregut, midgut, and hindgut, terminating in an anus;

✵ there is a closed Circulatory system, and many annelids possess respiratory organs—gills;

✵ excretory organs are of the metanephridial type—these are thin, paired tubules found in almost every segment, beginning with a ciliated funnel in the body cavity;

✵ The Nervous system consists of a circumpharyngeal nerve ring with a well-developed supra-esophageal ganglion (Brain) and a paired ventral nerve cord or ventral nerve chain;

✵ the majority of annelids are marine and freshwater dwellers, while a smaller portion live in moist soil. All of them lead a free-living lifestyle; only a few species are parasitic.

The phylum Annelida is divided into three classes:

1) Polychaetes, or bristle worms (Polychaeta). Numerous marine forms leading crawling or sedentary lifestyles. They feature well-developed parapodia with setae. Many species possess gills;

2) Oligochaetes (Oligochaeta). Parapodia and gills are absent; setae are smaller and fewer in number compared to polychaetes. Freshwater and terrestrial forms (including earthworms);

3) Leeches (Hirudinea). Highly modified oligochaetes. Predatory. Many are ectoparasites feeding on the Blood of fish, frogs, mammals, and other animals.