Introduction to Tapeworms.
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The Tapeworms (Cestoda).
The tapeworms are highly adapted to life as internal parasites inhabiting the gut of the host animal. The larger tapeworms are found in the gut of mammals, and smaller, more primitive types are parasites of fish. Tapeworms can reach a length of more than 30 metres, establishing them amongst the longest known invertebrates. (The free-living marine ribbon worms (Nemertines) can also reach this length).
They have no mouth or digestive system. Nutrients from the host digestive system are absorbed directly through the outer cuticle which is thickened and without cilia.
The head (scolex) is usually modified with hooks and/or suckers which fasten to the host gut wall and provide an anchor to prevent removal of the worm by host bowel activity. The body is divided into sections called proglottids containing little more than reproductive organs. The mature proglottids, gravid with eggs, detatch and are voided with the host faeces.
The rat tapeworm, Hymenolepis diminuta, has features typical of tapeworms as a group, and is shown in the pictures below. Since it is easily obtained from infected laboratory rats, it has been intensively studied, and probably more is known of this tapeworm species than of any other.
Under natural conditions, the rat is infected by eating infected grain beetles -- the intermediate host of H. diminuta. The beetles are infected by ingesting eggs contained in the faeces of infected rats.
Here is a link to a website containing more information and micrographs on H. diminuta.