36 PHYSIOLOGY OF THE INVERTEBRATA. 



(cfeca). However, in some of the Rotifrra, as, for example, 

 Notommata, the alimentary canal is blind or closed at the 

 posterior end ; and the males of some forms are entirely 

 devoid of any digestive apparatus. The whole of their brief 

 life is devoted to reproduction. 



(3) The Trcmatoda are all parasitic animals, having one or 

 more suckers upon the ventral side of the body, and behind 

 the month. The mouth leads into a muscular pharynx, which 

 opens into a more or less elongated oesophagus, and terminates 

 in a branched intestine. There is no anus. 



In Amphiptyclics and AmjjJdlina the alimentary canal is 

 entirely absent ; and on the authority of Professor V. J. Van 

 Beneden* it "becomes aborted in the adult Distoma Jilicolle." 



(4) The Oestoidea, or tape-worms, are examples of reversions 

 to a very low type of digestion, although in other respects 

 these animals are comparatively high in the zoological scale. 

 They live by the imbibition of partly digested food of the 

 animals whose intestines they infest. 



In the words of Professor Huxley, t " it is obvious that the 

 Ccstoidca are very closely related to the Trcmatoda. In fact, 

 inasmuch as some of the latter are anenterous, and some of 

 the former are not segmented, it is impossible to draw any 

 absolute line of demarcation that the Cestoidea are either 

 Trematodes which have undergone retrogressive metamor- 

 phosis, and have lost the alimentary canal which they 

 primitively possessed ; or that they are modifications of a 

 Trematode type, in which the endoderm has got no further 

 than the spongy condition which it exhibits in Convoluta 

 among the TurheUaria, and in which no oral aj^erture has 

 been formed ; or, lastly, it is possible that the central cavity 

 of the body of the embryo Taenia simply represents a blasto- 

 coele. If the Ccstoidca are essentially Trematodes, modified 

 by the loss of their digestive organs, some trace of the diges- 

 tive apparatus ought to be discoverable in the embryo tape- 



* Memoirc sur Its Vers Inte>itiiiaux (iS^S). 



+ The Anafovii/ of the Inrerfehrafed Animals, p. 213. 



