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  Cestodes, Biology tutorial

Introduction:

The cestodes comprise of two separate subclasses, the Cestodarians, parasites of fish and other cold blooded vertebrates. These are non-segmented parasites having only a single set of sexual organs. In disparity, the more renowned members of the Subclass Eucestoda are parasites of both warm and cold blooded vertebrates, comprising mammals like man. They look like a colony of individual animals in that their bodies are splitted into a sequence of segments (that is, the proglotids), each having their own complete set of internal organs. There might be many hundreds of such proglotids, resultant in the complete parasite encompassing a long and ribbon like body. The look of this long body is the origin for the common name for such parasites, the tapeworms. The general names of such parasites are often derived from their intermediate hosts, ingestion of which outcomes in their infection, example: the Beef, Fish and Pork Tapeworms.

The Adult Parasite:

The adult tapeworm's body might be categorized into three regions:

1) The Scolex:

This is the 'head' and attachment organ of the parasite. There are basically four kinds of scolex, by which the tapeworm might be taxonomically categorized. 

a) No special attachment organs:

The scolices of a few tapeworms of the order Caryophyllidea (that is, parasites of freshwater fish) encompass no special attachment organs.

b) A Bothria:

This is comprised of a pair of shallow, lengthened and weakly muscular grooves. Tapeworms of the order Pseudophyllidea are equipped by Bothria on their scolices.

c) A Bothridia:

These are wide, leaf like muscular structure, showing a high degree of variation. Some Bothridia are sessile, a few are stalked, while others are hooked by accessory suckers. Tapeworms of the order Tetraphyllidea and others are equipped by means of Bothridia.

d) Acetabulate Suckers:

Tapeworms of the order Cyclophyllidea are equipped by means of four Acetabulate suckers.

2) The Neck:

This is the region of proliferation of the parasite, from which the proglottids of the strobila grow. 

3) The Strobila:

This is comprised of a sequence of proglottids. Each and every proglottid includes a complete set of female and male reproductive organs; however these organs generally mature at different rates. Generally the male organs build up before the female organs and degenerate before the female organs mature. The big, gravid proglottids at the posterior end of the tapeworm are full of developing or in the great terminal proglottids, mature eggs. 

The Cestode Tegument:

The associated cestodarians which as well belong in the cestodes encompass a tegument which appears to be intermediate with that of the monogeneans and eucestodes. This is the other piece of evolutionary evidence which points out a monogenean origin for the tapeworms. In this situation the surface of the cestodarian tegument is covered by many microvilli, identical in form to the eucestode microtriche, however lacking the electron dense cap seen in such parasites.  The cestode tegument is a syncytial layer, exhibiting numerous features typical of that found in other parasitic platyhelminthes.

There are though, a number of differentiating features present in such parasites. On the very outer surface of the tegument a surface glycocalyx is seen to cover the external plasma membrane. Beneath this glycocalyx, a characteristic attribute of the eucestode tegument is the presence of many microtriches (Mt), long spine like processes which are though a highly modified form of micovilli. Each and every microthrix consists of a hard, pointed, electron dense cap that is seperated from the rest of the microthrix through a crescent shaped membranous cap. The mirotriches are thought to serve up two functions. First of all, the tapeworms don't possess a gut and should absorb all of their nutrients across the surface tegument. The microtiches very much raise the surface area of the parasite, and can be seen as an adaptation to maximize the amounts of nutrients accessible to the parasite. This is supported by the finding of microtubles in the shaft of the microtriches. Secondly the spine likes the character of the microtriches almost certainly assist the parasite maintain its position in the gut. This can be more evidently seen by comparing the microtriches found in various regions of the body of the parasite.

Larval Metacercaria:

The Larval Cestodes:

1) The Cestodarians:

The cestodarians larvae or lycophore are free swimming, being covered in the cillia. They encompass a set of ten hooks at the extreme anterior of the body, therefore differing from the larval eucestodes that are equipped having three pairs of hooks. Anteriorly they are armed by the penetration glands. The bodily form of such larvae bears a marked resemblance to the larvae of the Trematodes, like the miracidium in the Digeneans, and the larval monogenean, the oncomiracidium.

2) The Eucestodes (Tapeworms):

The eggs of Cyclophyllidean and Pseudophyllidean cestodes vary considerably. The egg of the pseudophyllidean tapeworm closely looks like that of the Trematodes, having a thin shell wall, and an operculum, which on hatching opens to reseals the free swimming larvae. This describes the close relationship among the two main groups of platyhelminthes parasites. In contrary, the egg of the cyclophyllideans tapeworms is much different, having a much thick, resistant egg shell having no operculum.

Metacestodes:

The larval phases of a tapeworm, comprising the metamorphosis of the oncosphere to the first evidence of sexuality in the adult worm, differentiation of the scolex and starting of proglottid formation; it comprises the procercoid and plerocercoid phases of the pseudophyllid cestodes, and the cysticercus, cysticercoid, coenurus, and hydatid phases of the cyclophyllidean cestodes.

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