Cestoidea

Cestoidea, one of the classes of flat worms or Platyelminthes. They are unisegmental, or else appear to be falsely segmental, as the body is composed of many joints known as "proglottides." They are all parasitic, and therefore are degraded: they have no organs of special sense, or definite digestive tract. The body or "strobila," as the full sexual form is called, consists of three main parts: the head or proscolex, the neck and the joints or proglottides. The former contains the hooks or suckers by which the worm attaches itself to the wall of the intestine of its host (the animal in which it is parasitic). The head may have four suckers and a circle of hooks, as in the common Taenia or tapeworm; or may have four suckers and four processes, or "proboscide" armed with hooks; or it may be provided only with a pair of sucking grooves. The head and neck form the scolex. The neck is short, and normally gives origin to a series of "proglottides," or joints which are shed when mature. Each joint contains a complete set of generative organs and excretory vessels, also a pair of lateral nerve cords, which run down the whole length of the body from the single anterior nerve ganglion. But in some cases the proglottides are not well defined, as in the genus Bothriocephalus, which infests the dog and horse, etc.; while they are not developed at all in the family Caryophyllaeidae, which inhabit certain fish (Cyprinoids). The Cestoidea are hermaphrodite, each ripe joint containing a set of both male and female organs: these are somewhat complex. The most striking feature in this class is the life history. The eggs are swallowed by some animal, generally one that feeds on vegetable material; when in the stomach of this, the gastric fluid dissolves the strong leathery coat in which the eggs are protected, and the embryo absorbs the digestive juices. The head is then developed, and by its hard jaws burrows its way through the walls of the stomach or intestine, and settles in the tissues of the host; it there becomes "encysted," i.e. it enters upon its resting stage, inclosed in a thick case. In this condition it remains till its host is eaten by some carnivorous animal. The cyst having been ruptured or dissolved, the larval worm continues its development. The scolex, composed of the head and neck, is first formed, and by the suckers or hooks on the head attaches itself to the wall of the intestine: it absorbs the food material prepared by the host, and soon develops the joints or proglottides. These increase in size from before backward; new ones are formed immediately behind the head: the oldest and largest ones are shed when mature.

The origin of the Cestoidea is a somewhat obscure problem. It has been thought that the proglottides are a case of serial gemmation comparable to that of some Tunicates, etc., and that the whole life history is but a case of Alternation of Generations (q.v.). This view involves the idea that the proglottides are to be considered to some extent as individuals. That however, can hardly be the case, on account of the continuity of the various body-systems, notably the nervous system, from the head right through the proglottides. It seems more probable that the scolex and proscolex are both to be regarded as non-sexual forms, which only become sexual under the more favourable conditions of food supply, etc., offered by the second host. The scolex and proscolex in fact may be only organs of adhesion, a view urged by Moniez. The latter view, which does not apply the phenomenon of alternation of generations, gains support by the facts that in one case (Ligula) the joints are developed in the first host, while in another (Archigetes) the whole life history is completed in the one host - the small red river worm Tubifex; this latter case is of interest, moreover, as the only Cestoid parasitic on an invertebrate. There are six families included in the Cestoidea.