Annelids

Annelids (segmented worms) are extremely successful and have invaded terrestrial, freshwater, and marine habitats. They include familiar forms such as earthworms and leeches and many diverse marine organisms. The segmentation characteristic of this phylum extends to the interior of the animal and is more complex than the annulations of the rotifera and nematodes. Metamerism, a repetition of similar body parts, is related to the segmentation and is probably an adaptation for locomotion.

• Class Oligochaeta. Members of the Class Oliogochaeta include earthworms and many freshwater forms. Metamerism is well-developed and most apparent in the posterior segments. The head is degenerate with few obvious sensory structures (owing to their burrowing habits). Oligochaetes have few setae (spine-like appendages; oligo="few", chaeta="setae"). The setae are the spine-like hairs extending from the body. Members of this class also possess a reproductive structure called the clitellum (a reproductive structure). Oligochaetes are monoecious.

FIGURE 1. MAJOR INTERNAL ORGANS OF AN OLIGOCHAETE (Lumbricus terrestris).

Figure 1B. Cross section of an earthworm

Figure 1C. Earthworm nephridium

Earthworm Cross Section and Dissection	Longitudinal Section	Cross section	Cross Section
Dissection 1	Dissection 2	Earthworm Metanephridia	Earthworm nervous system

• Class Polychaeta. Polychaetes are the most structurally diverse of the annelids and are exclusively marine. They may be sedentary or free moving. The free-moving forms have the more typical polychaete body plan while the sedentary and tube-dwelling forms are often bizarre in the extent of their modifications. Polychaetes have many setae (poly="many") and gill- or fin-like appendages called parapodia. In some species the setae may cover the entire body which gives it a furry appearance (sea mouse, Aphrodite). Others have hollow poison-tipped setae used for defense (as in the "fire worms"). The parapodia may be relatively simple (Nereis), or extremely complex and varying from segment to segment (Chaetopterus). The heads of polychaetes are usually more complex than those of oligochaetes. Eyes, tentacles, palps, and pincer-like jaws are the rule. The heads of plume and feather duster worms are endowed with tentacles to capture prey. Polychaetes lack a clitellum and are mostly dioecious.

Figure 2. Anterior segments of the clamworm, Nereis (Phylum Annelida, Class Polychaeta).

Feather Duster Worm	Plume Worm	Chaetopterus
Aphrodite (sea mouse)	Nectarus	Nectarus parapodium

• Class Hirudinea. This freshwater class of annelids is commonly called leeches. They usually have an anterior sucker surrounding the mouth and a ventrally-facing posterior sucker. Like earthworms they are monoecious, but asynchronously so (individuals switch between being male or female, but are never both at the same time). Although leeches have a reputation as being blood suckers, most are predacious.

Leeches