TAPEWORMS. The Cestodes or Tapeworms form a class of purely endoparasitic Platyelmia, characterized by their elongate shape, segmented bodies, and the absence of a digestive system.
Those Cestodes which possess no very distinct organ of attachment (such, for example, as Gyrocotyle) have no distinct ganglionic thickening more pronounced at one end of the body than at the other; and as these are forms which have retained more primitive features than the rest, and show closer affinity to the Trematodes, it seems highly probable that the complicated nervous thickening found in the scolex, and often compared with the " brain " of other Platyelmia, is a structure sui generis developed within the limits of the sub-class.
In fact, the whole history of the Platyelmia is marked by a great specialization of the reproductive evolutionary history, accompanied by a simple somatic line of evolution.
From what we know of the Platyelmia, however, it is more probable that the two are quite independent and have been evolved separately.
As in other Platyelmia the elements of this tissue undergo the most varied differentiation.
PLATYELMIA, a phylum of the animal kingdom which comprises three classes, the Planarians, Trematodes and Cestodes.
The chief features which Platyelmia possess in common are the following.
The Platyelmia are hermaphrodite and the reproductive organs are complex.
In the majority of Platyelmia the primitive ovary becomes divided into fertile and sterile portions, i.e.
The inter-relationships of the three members of the Platyelmia are of a more doubtful nature than is the unity of the phylum.
As the Turbellaria (Planarians) are the most primitive division of the Platyelmia, the problem of the affinities of this phylum resolves itself into that of the relationships of the Turbellaria.
- Recent discussions of the affinities of the Platyelmia will be found in (I) A.