THE DEVELOPMENT OF THE CESTOIDEA. 187 



Cystic Form. Cestoid Foem. 



Cysticercus fasciolaris. Tmnia crassicollis. 



(Liver of Eats and Mice) (Cat) 



Ccenurus cerebralis. Taenia ccenurus. 



(Sheep's brain) (Dog) 



Echinococcus veterinorum. Tmnia Echinococcus. 



(Liver of Man and of (Dog) 

 domestic Ungulata) 



The embryo of Tmnia cucumerina passes, in the body of 

 the Dog-louse (Trichodectes canis), into a Cysticercoid, or 

 minute unjointed and sexless Taenia, without any terminal 

 dilatation. The dog devours the louse and the Cysticercoid 

 becomes a Taenia cucumerina in his intestine. The eggs of 

 the Taenia, contained in fasces adherent to the hair of the 

 dog, are in turn devoured by the louse, and thus the " vicious 

 circle " of parasitism is maintained. 



The cystic Tetrapliyllidea frequent osseous fishes, their 

 sexual maturity being attained in the bodies of Plagiostomes. 

 The head is provided with four suckers or lobes, which may 

 be stalked and unarmed, as in Echeneibothrium, or furnished 

 with hooklets as in Acanthobothrhtm ; while, in Tetrarhyn- 

 chus, four proboscidiform tentacles, thickly set with hooklets, 

 are retracted into sheaths alongside of the suckers (Fig. 46). 



The Diplxyllidea have two suctorial disks, two armed 

 rostellar prominences, and a collar of hooklets on the neck. 



The migrations of the Pseud ophyllidea are chiefly from 

 fishes and amphibians to w^ater-birds, one genus (Bothrio- 

 cephalus) containing species which enter the human body, prob- 

 ably in the flesh of fresh-water fishes. The head has neither 

 suckers nor lobes, but is deeply grooved on either side. In 

 Bothriocephalic the genital apertures are in the middle of 

 each segment. The embryo is ciliated, and swims actively in 

 water. Recent experiments tend to show that the develop- 

 ment of the embryo in this genus may take place directly, or 

 without the intervention of a Cysticercus stage. 



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 demarkation between 

 the two groups. It would appear that the Cestoidea are 

 either Trematodes which have undergone retrogressive met- 

 amorphosis and have lost the alimentary canal which they 

 primitively possessed, or that they are modifications of a 



