Worms: Flatworms: Tapeworms (Cestoda). Introduction with photomicrographs.

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.

Hymenolepis.

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.

	An entire rat tapeworm. The folded worm as shown in the picture is 100mm across. It has been photographed in a glass dish with incident lighting from the top against a black background, using a Nikon 35mm camera with macro lens. Click the link to see the photographic setup used for this picture.
	A closer look at sections of the same worm. The larger section at the left is of the mature portion close to the end. Rheinberg, x35.
	The head, or scolex, of Hymenolepis. Although it is not clear in this picture, the scolex has no hooks, but possesses four suckers which enable it to embed securely in the wall of the rat's intestine. The production of new proglottids occurs in this region by a process of budding. Rheinberg, x70.
	The terminal proglottid of the specimen of the previous pictures. In these mature or "ripe" sections, the ovaries and testes have degenerated, and almost the entire proglottid is occuppied by a branched uterus filled with eggs, some of which are visible through the thickened cuticle. A single egg is seen in profile on the right. Rheinberg, x70.