36 PHYSIOLOGY OF THE INVERTEBRATA. 



(cseca). However, in some of the Botifera, 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 Trematoda are all parasitic animals, having one or 

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

 the mouth. 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 Amphiptyches and Aniphilina the alimentary canal is 

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

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



(4) The Cestoidea, 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 

 Cestoidea are very closely related to the Trematoda. 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 farther 

 than the spongy condition which it exhibits in Convohta 

 among the Turhellaria, and in which no oral aperture has 

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

 of the body of the embryo Ta;nia simply represents a blasto- 

 coele. If the' Cestoidea 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- 



* Mimoire sur les Vers Intestinaux (1858). 



+ Tlie Anatomy of the Invertehrated Animals, p. 213. 



