Aims and objectives

This exercise is designed to demonstrate:

1. The general morphology of the protozoan parasite Monocystis.

2. The general morphology of the parasitic and free-living stages of the nematode parasite Rhabditis.

Introduction

Earthworms are common terrestrial invertebrates from the phylum Annelida. They are exploited by a number of protozoan and nematode parasites and act as intermediate hosts for many parasites of birds and mammals. Two common parasites are Monocystis (Protozoa, Sporozoa) and Rhabditis (Nematoda); these infect earthworms only.

Monocystis is the commonest protozoan genus to infect earthworms, but another nine genera have been recorded in Britain. The growing form (trophozoite) and reproductive forms occur in the seminal vesicles of the earthworm; the reproductive forms (sporocysts – contained within larger cysts or spores) also enter the body cavity.

The nematode parasite Rhabditis also infects earthworms but only in its larval phase. The parasitic third-stage larvae occur free in nephridia of the earthworm, encysted in the body wall, or encapsulated in the coelom. Adult nematodes develop only when the worm dies, when the larvae begin feeding on the bacteria that break down the tissues. Adult Rhabditis is therefore a free-living organism. Several species of Rhabditis are involved, of which the most common is R. maupassi, but they are difficult to differentiate.

Earthworms have a body cavity (a true coelom), a well-developed blood system and the capacity to defend themselves against some types of invaders.