Cestodes, external surfaces

For cestodes, uptake via the external surface, the syncitial tegument, is the sole means of obtaining nutrients, as they do not have an alimentary tract. This absence of an alimentary tract is a striking feature of all cestodes. In this respect, it should be realized that all cestodes are parasites. 

The external surfaces of parasitic helminths, termed the tegument in cestodes and trematodes, and the cuticle in nematodes, are adapted to serve a wide range of biological functions. Though most research has focused on structural aspects or their roles in immune evasion (Chapter 16) and nutrient absorption, the external surfaces of helminths also serve important roles in locomotion, excretion and regulation of electrochemical and osmotic gradients. 

The absence of a gut in cestodes has simplified interpretation of functional properties of the external surface. The gastrodermis in trematodes and the intestine in nematodes are composed of cells with microvilli which greatly amplify the surface area for absorption. However, too little is known about transport across the internal surfaces of trematodes or nematodes to draw definitive conclusions about their importance to the parasites, or the extent to which they resemble analogous processes in other organisms. 

Although cestodes have received less attention than the trematodes or nematodes, discussion begins with these parasites because of their anatomical simplicity. The absence of a gut simplifies the interpretation of functional properties of the external surfaces (reviewed in refs 2 and 4). 

Gross and microscopic anatomy. All interactions between the cestode and its environment occur across the external surface, or tegument. In addition to the functional properties associated with this tissue in other helminths, the cestode surface must also be structurally adapted to perform all functions normally associated with intestinal tissue. Indeed, the cestode body plan has often been conceptualized as an inside-out intestine . Consequently, research on the cestode surface has been somewhat biased toward features that account for its role in nutrition. 

The rigid cuticle that is the external surface of nematodes is quite distinct from the tegument of cestodes and trematodes. Nematodes also possess a functional gut that ends in a muscularly controlled rectum. Although the various niches occupied by nematodes have led to evolutionary surface adaptations particularly suited for specific environments, the general features of their surface biology are well conserved. 

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