BASIC BODY PLAN OF A CESTODE

INTRODUCTION

The cestodes consist of two separate subclasses, the Cestodarians, parasites of fish and other cold-blooded vertebrates. These are non-segmented parasites, with only a single set of sexual organs. In contrast, the more well-known members of the Subclass Eucestoda are parasites of both warm and cold-blooded vertebrates, including mammals such as man. They resemble a colony of individual animals in that their bodies are divided into a series of segments (the proglottids), each with
their own complete set of internal organs.

 There may be many hundreds of these proglottids, resulting in the complete parasite having a long, ribbon-like body. The appearance of this long body is the origin for the common name for these parasites, the tapeworms. The common names of these parasites are often derived from their intermediate hosts, ingestion of which results in their infection, e.g. the Fish, Beef and Pork Tapeworms.

  Alternatively, they may be named after the definitive host that the adult parasites are usually found in. For example, the Rat tapeworm H. diminuta and the Dog Tapeworm Dipylidium caninum. The study of the morphology of the cestode body may be divided into two distinct areas. Firstly, the morphology of the adult cestode (the tapeworm) and secondly the morphology of the cestode larvae, or Metacestode.

The Scolex

This is the "head" and attachment organ of the parasite. There are four main types of the scolex, by which the tapeworm may be taxonomically classified.

a) No particular attachment organs The scolices of some tapeworms of the order Caryophyllidea (parasites of freshwater fish) have no individual attachment organs. (NB. Some authors do not
recognise this taxonomic order, placing these parasites within the Pseudophyllidea).


b) A Bothria - This is composed of a pair of shallow, elongated, weakly muscular
grooves. Tapeworms of the order Pseudophyllidea are equipped with bothria on
their scolices.

The bothria of Pseudophyllidea

c) A Bothridia - These are large, leaflike muscular structure, exhibiting a significant
degree of variation. Some bothridia are sessile, some are stalked, while others are
hooked with accessory suckers. Tapeworms of the order Tetraphyllidea and others
are equipped with bothridia.

d) Acetabular Suckers - Tapeworms of the order Cyclophyllidea are equipped with four acetabular suckers. Parasites in this order may also have additional features at the apex of the scolex such as;
 - Glandular areas
 - Protrusible suckers
 - Suckers armed with hooks
 - Hooks (e.g. Taenia)
 - A rostellum, an eversible muscular proboscis, often covered
 with hooks (e.g. Hymenolepis, Echinococcus, Dipylidium)
 - A Myzorhynchus (a protrusible muscular mass).

The Neck
 
This is the area of the proliferation of the parasite, from which the proglottids of the strobila grow.


The Strobila
 
This is composed of a series of proglottids. Each proglottid contains a complete set of male and female reproductive organs, although these organs usually mature at different rates. Often the male organs develop before the female organs and degenerate before the female organs age. The large, gravid proglottids at the posterior end of the tapeworm are full of developing, or in the extreme terminal proglottids, mature eggs.

The Cestode Tegument

The related custodians that also belong within the cestodes have a tegument that appears to be intermediate with that of the cestodes and monogeneans. This is another piece of evolutionary evidence that indicates a monogenean origin for the tapeworms. In this case, the surface of the custodian tegument is covered with numerous microvilli, similar in form to the cestode microfiche (see below), but lacking the dense electron cap seen in these parasites.
The code tegument is a syncytial layer, showing many features typical of that found in other parasitic Platyhelminthes.

  There are, however, several distinguishing features present in these parasites. On the very outer surface of the tegument, a surface glycocalyx is seen to cover the outer plasma membrane. Below this glycocalyx, a characteristic feature of the cestode tegument is the presence of numerous microfiches (Mt), long spine-like processes that are in fact a highly modified form of microvilli. Each microthrix has a hard, pointed, electron-dense cap which is seperated from the rest of the microstrip by a crescent shaped membranous cap.

  The mirotriches are thought to serve two functions. Firstly, the tapeworms do not possess a gut and must absorb all of their nutrients across the surface tegument. The microfiches significantly increase the surface area of the parasite and can be seen as an adaptation to maximise the amounts of nutrients available to the parasite. This is supported by the finding of microtubles in the shaft of the microfiches. Secondly, the spine, like the character of the microfiches probably help the parasite maintain its position in the gut. This can be more clearly seen by comparing the microfiches found in different regions of the parasite’s body.

  It has been noted in many species that the microfiches found covering the scolex, the attachment organ of the parasite, were much longer than those covering the strobila, and in some species show particular adaptations. For example, the microfiches covering the strobila of E. granulosus have been found to show curved hooks or sometimes even barbs. Below the layer of microtriches the main syncytial layer of the tegument is found. This has been seen to contain numerous vesicles and membrane bound, electron dense rod-like structures,refered to as disc-shaped bodies (Db). Finally, many mitochondria, mainly in the distal region of the tegument, may be seen. These are unusual in that they do not have many cristae, reflecting the anaerobic metabolism of the organism.

   The tegumental nuclei are however not located in this outer layer but are found within subsegmental cell bodies (StC), situated beneath the circular (Cm) and longitudinal muscle (Lm) layers, embedded within the parenchymal tissues (P) and mesenchymal musculature (M). These subsegmental cell bodies also contain other cellular elements such as Golgi apparatus and lipid inclusion bodies (L) which are connected to the outer syncytium and areas of glycogen storage (Gs) by long protoplasmic extensions (Pe). The location of these critical cellular elements away from the outer surface of the parasite, exposed to immunological attack by the parasite's host, is an essential adaptation to a parasitic lifecycle adopted by all of the parasitic Platyhelminthes.

  The parenchymal tissues are similar to those of the trematodes and fill the spaces between the internal parasites organs (all cestodes and other Platyhelminthes being acoelomate organisms).
These tissues are a syncytial network formed by anastomosis of mesenchymal cells, with spaces filled with carbohydrate-rich parenchymal fluid.