126 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



of the animal is lengthened out into the shape of a 

 narrow ribbon. Of the primitive lateral row of 

 plates, four are represented by mere vestiges, and 

 the other four, which continue to be functional, are 

 situated, closely coupled together, on the two edges 

 of the animal. 



2. Worms. We will next take in succession the 

 Plathelminthes or flat worms, the Rotifers, the 

 Nemathelminthes, or round worms, the Annelids 

 and the Gephyreans. 



Among the Plathelminthes, the group of Cestodes 

 contains the common tape- worm of man (Tcenia 

 solium). In the course of its parasitic existence 

 this worm has undergone considerable morphological 

 changes. The digestive tube is lacking, and the 

 whole nervous system has become greatly simplified. 

 The degeneration of the nervous system is not, 

 however, complete, for important vestiges still per- 

 sist. The degeneration of the digestive tube is 

 much more thorough. In Tcenia solium it is alto- 

 gether absent. In species closely allied to the Tcenia 

 some slight vestiges of the digestive apparatus yet 

 remain. The head or scolex of some species of 

 Tetrarliynchus contain glandular cells which have 

 been homologized with the salivary glands of other 

 flat worms (Trematodes). In other species of Tetra- 

 rhynchus there is a rudimentary organ which repre- 

 sents the oval sucker of the Trematodes, and in 

 Anthrocephalus elongatus the orifices of the salivary 

 glands are in the region of this vestige of the 

 digestive tube. 



