Phylum Annelida

Phylum Annelida

Annelids are the true "segmented worms" — the first phylum in this survey to show metamerism, the division of the body along its length into a linear series of repeating segments (metameres), each segment typically containing its own set of muscles, nerve ganglia, excretory organs and, often, reproductive structures, in addition to a shared gut and nerve cord running the length of the body. Metamerism allows greater precision of movement (each segment's muscles can contract semi-independently) and provides a degree of built-in redundancy, since damage to one segment need not disable the functioning of the whole animal.

Annelids are also true coelomates, with a spacious, fluid-filled coelom that acts as a hydrostatic skeleton against which the body-wall muscles work during locomotion.

The phylum is divided into three classes:

• Class Polychaeta — chiefly marine annelids bearing numerous chitinous bristles (setae) grouped on paddle-like lateral extensions called parapodia, which project from each segment and are used as the main locomotor structures for crawling and swimming; polychaetes typically have a well-developed head with sensory appendages, and the sexes are usually separate. Nereis (the ragworm) is a classic example showing parapodia used actively for locomotion.
• Class Oligochaeta — annelids with relatively few setae per segment (unlike the numerous setae of polychaetes) and lacking parapodia; the head is reduced and lacks the elaborate sensory appendages of polychaetes; most oligochaetes are hermaphroditic (each individual has both male and female reproductive organs) and undergo direct development without a larval stage. The earthworm, Pheretima, belongs to this class and is examined in detail in the following section.
• Class Hirudinea — the leeches, mostly ectoparasitic or predatory annelids, distinguished from the other two classes by the presence of anterior and posterior suckers used for attachment and locomotion (a looping movement alternating between the two suckers), the near or complete absence of setae and parapodia, a fixed and reduced number of body segments, and a coelom that is largely reduced and filled with a spongy connective tissue (rather than remaining a large, open fluid-filled cavity as in Polychaeta and Oligochaeta). Hirudinea are also hermaphroditic, like Oligochaeta, but are set apart from both other classes chiefly by their sucker-based body plan adapted for a parasitic or predatory habit — for instance, on the medicinal leech (Hirudo). …