(Underlined words are defined in the glossary)

Several species of Monocystis are parasitic in the seminal vesicles of the earthworm. They belong to the class Sporozoa and are placed in the order Gregarinida.

The adults, or mature trophozoites, are commonly to be found within the seminal vesicles of the worm. There is a thick pellicle beneath which, in the plasmagel, are longitudinal myonemes. In the granular plasmasol there are paramylum granules and an ovoid nucleus with a prominent nucleolus. Locomotion of the parasite is characterised by contraction of the myonemes during wriggling; it is known as gregarine motion. (Fig. 1 right).

The parasite feeds on the cytoplasm of a sperm morula by extruding enzymes and absorbing the digested products through the pellicle. It will frequently move to another morula and consume the cytoplasm, before it is fully-grown. Often numerous sperm tails adhere to the pellicle, giving the Monocystis a ciliated appearance. (Fig. 2 left).

Excretion and respiratory activities are carried out by diffusion.

When two mature adults come together they secrete a common wall and form a conjugative cyst. Within the cyst mitotic and meiotic divisions occur. There is no further development until another earthworm, which first entails the liberation of the cysts into the soil, swallows the cysts. This latter is accomplished by birds, or other animals, eating the earthworm. The cysts are not digestible and are voided with the faeces. Another worm now swallows the cyst, the cyst coat is digested and the freed sporozoites migrate to the seminal vesicles by boring through the gut wall into the coelom of the earthworm. Cysts may live in the soil for a considerable period.

The majority of earthworms are infected by the parasite. In fact, in many years, I have not found the absence of the parasite in the seminal vesicles of any worm examined. Quite often there is a heavy infestation. Despite this, the presence of the parasite seems to cause little inconvenience to the worm. Sperm are produced in such quantity that the destruction of quite large amounts has little effect on the reproductive capacity of the worm.

Smear Preparation of the Contents of the Seminal Vesicles.

For this preparation the worm should be killed with chloroform and opened dry. A mid-dorsal incision is made, about segments 9 - 15. This will clearly show the white seminal vesicles. These are cut off and placed in a watch-glass. The material is covered, with about five times its bulk, with 0.75% saline, and is then teased thoroughly with needles, to release the contents of the seminal vesicles. A drop of the milky fluid obtained, is placed on a cover-glass, dried by warming and fixed in alcohol. This is then stained with Ehrlich's haematoxylin and again dried by warming. The cover-glass is placed on a microscope slide, in the centre of which a drop of Canada Balsam has been placed, and is then examined microscopically. Apart from the presence of the parasite, all the developing stages of the spermatozoa (the morula, resembling a blackberry) will be seen. (Fig. 3 above right).

Useful reference for staining technique:
Biology Staining Schedules by R.R. Fowell. H.K. Lewis & Co. Ltd.(London)

Another useful reference is Dissection Of The Earthworm. Whitehouse & Grove. University Tutorial Press Ltd.

Comments and feedback to the author Mike Morgan are welcomed.

Also see Micscape article 'Worm dissection'.

Glossary

Morula: solid round mass of cells, resulting from the division of an egg.

Myoneme: a muscle fibre.

Paramylum: stored carbohydrate, resembling starch.

Pellicle: relatively stiff, thin surface layer covering the cell.

Plasmagel: jelly-like state of the outer cytoplasm.

Plasmasol: the fluid inner cytoplasm.

Seminal Vesicle: the organ that stores sperm in invertebrates, such as the earthworm.

Sporozoite: the minute spore, developed by Sporozoans, which infect the
host animal.

Trophozoite: a growing stage in the life cycle of some Sporozoan parasites, when they are absorbing nutrients from the host.

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Published in March 1999 Micscape Magazine.

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