Attachment organs are probably not as important for the digenea as the monogenea since the location of their attachment sites means that they do not experience the same degree of physical buffeting that say a monogenian clamped on to the gill lamellae of a fish would experience. However, attachment is still important and most digeneans have a large ventral sucker (acetabulum) and a small oral sucker surrounding the mouth for this purpose (Meglitsch 1972).Most digenians feed on blood (Haematophagy) and in several polyopisthocotyleans this has necessitated modification of the digestive system in order to deal with extracellular accumulations of haematin pigment. Some monopisthocotyleans are skin browsers, so they possess a protrusible pharynx and feed in a similar way to predatory turbellarians (Halton 1997).

 The miracidium is the free-swimming larva that hatches from the digenean egg. It is in many ways similar in structure to the monogenian oncomiracidium as it too needs to sense and find a new host. Locomotion is provided by ciliated epidermal plates.At the anterior end of the larvae is a nipplelike projection (terebratorium) containing sensory organs and the apical and penetration gland openings. The terebratorium acts as a sucker whilst secretions from the penetration glands help the miracidium penetrate the outer surface of the first intermediate host (a mollusc). Eye spots are also present on the dorsal surface. (http://www.path.

cam.ac.uk/~schisto/SchistoLife/Miracidium.html).The cercaria is the other free-swimming digenean larva.

It emerges from the mollusc intermediate host and needs to locate the next host so has a tail for swimming and various sensory organs. The body of the cercaria contains penetration gland cells to facilitate entry into the next host and cystogenous gland cells used to secrete a cyst wall for the metacercarial stage. Oral and ventral suckers similar to those of the adult aid initial attachment of the cercaria to the host before penetration proceeds (http://www.

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path.cam.ac.uk/~schisto/OtherFlukes/Platy_Gen/Platy_Teg.html).The cestodes have become adapted for parasitic existence in the vertebrate gut. Here, among many other hazards, they have to withstand attack by digestive enzymes and alkaline secretions (http://www.

uel.ac.uk/biosciences/res/year2/BS257/Platyhelminthes/Platyhelminthes.htm).

However, one significant advantage is that they are surrounded by pre-digested food and can draw nourishment directly from it through the tegument.Therefore, they have no need of a gut, and indeed do not have one, relying instead entirely on the host for their nutrition. Cestodes have a syncytial tegument and nuclei within sunken epidermal cell bodies similar to that seen in the other parasitic platyhelminthes but with some additional unique features. The eucestode tegument bears modified microvilli with electron dense tips (microtriches).

These greatly increase the surface area available for nutrient absorption, essentially allowing the tegument to function in a similar way to the mucosa of the vertebrate gut. They also help in attachment to the intestinal wall of the host. The densely packed microvilli on the outer surface of cestodarians resemble mitrotriches but lack electron dense tips.

(http://www.path.cam.ac.uk/~schisto/Tapes/Tapes_Gen/Tapes_Teg.

html).A surface layer of polysaccharide (glycocalyx) coats the cestode tegument and may help avoid possible damage from the phagocytic and immune response of the host and act as a reservoir for nutrients by binding certain molecules before passage through the cestode tegument. The glycocalyx may also facilitate attachment (http://www.

bact.wisc.edu/Bact330/lecturead). The tegument of cestodes also contains glands that emit compounds that hinder the digestive enzymes of the host by reducing the pH of the environment to that which suits the parasite and not the enzymes (http://www.uel.ac.

uk/biosciences/res/year2/BS257/Platyhelminthes/Platyhelminthes.htm).The elongated string of repeated segments (proglottides) and intricate attachment organ, the scolex, is an efficient design for life spent in the vertebrate intestinal tract (Barnes et al.

1993). This permanently parasitic existence has resulted in the loss of not only the gut but also special sense and locomotor organs. However, this does not mean that cestodes are completely without senses. Many of the free nerve endings that occur on their bodies are probably sensory and it has been shown that the club-shaped neurosensory endings are tactile (Meglitsch 1972). The cestodes are not totally sedentary but are able to move by muscular contraction. (Laverack and Dando 1987).There are four different kinds of scolices: with no attachment organs, with elongated, weakly muscular grooves (Bothria) (Fig.

2.), with broad leaflike muscular structures (Bothridia) (Fig. 3.) or with four acetabulate suckers which may additionally have various arrangements of hooks, suckers, glandular areas and a rostellum armed with hooks (Fig.

4.) (http://Path.cam.ac.uk/~scisto/Tapes/Tapes_Gen/Tapes_Teg.html). Intricate attachment organs are necessary so that the cestode avoids being dislodged by the incessant buffeting it must experience as peristaltic waves and food pass down the gut (Barnes et al. 1993).

The string of proglottids are produced by budding, usually in the germintive zone just behind the scolex, and form the main body (strobilla). As each new proglottid is produced, the other proglottids move down the chain. Individual proglottids contain both male and female reproductive organs and one proglottid may self-fertilise another by folding of the body or alternatively may be cross-fertilised from the proglottid of another strobila (Hickman et al. 1998).Chances of finding a new host are slim and proliferation of the gonads in this way greatly increases reproductive output.

The more larvae produced the greater the chances of any single larva finding its way to the next host (Grzimek 1974). The free-swimming larvae of the cestodes show similar adaptations for locating and penetrating the next host as the free-swimming larvae of monogeneans and digenians.The external as well as internal surfaces of many types of animals provide a surprising number of niches. The synctial tegument is probably the single most important adaptation that has allowed the parasitic platyhelminthes to so effectively exploit these niches. The locomotory cilia and well developed sense organs are perhaps the structural elements most fundamental to survival of the free-living turbellaria. The same could also be said of the free-swimming monogenean, digenean and cestodean larvae.

Despite the relatively simple structure of the platyhelminthes, the morphological adaptations to lifestyle reviewed above are quite remarkable. They result in a group of animals very well equipped for a predatory life or alternatively to take advantage of the unseen and seemingly hostile but fertile niches provided by the bodies of other organisms. (1947 words).ReferencesBarnes, R.S.

K., Calow, P. and Olive, P.J.W., 1993.

The Invertebrates. 2nd edition. London: Blackwell Science.Buchmann, K.

and Lindenstr�m, T., 2001. Interactions between monogenian parasites and their fish hosts.

International Journal for Parisitology, 32, 309-319.Cambridge University Schistosomiasis Research Group, n.d., The Cestode Tegument (online). Available from:http://www.path.

cam.ac.uk/~schisto/Tapes/Tapes_Gen/Tapes_Teg.html (Accessed 27 Feb. 2003).