362 



THE DEVELOPMENT OF CESTODES. 



point of connection is seen as a narrow mouth-like opening, near 

 which, in successful preparations, the embryonic hooks may be clearly 

 seen. Since this is just the position at which the rudiment of the 

 head first appears, one might suppose that the latter is formed also in 

 the bladder- worms proper from the anterior hook -bearing segment of 

 the embryo. 



Fio. 209. Cysticercu* arionis with head retracted (A), and protruded (). ( x 50.) 



The formation of the head has not, however, as yet been directly 

 observed in Cysticercus arionis. This is the more to be regretted since the 

 adult head is by no means a mere repetition of the ordinary Cysticercus 

 type, but has its anterior end (with rostellum and hooks) always 

 turned towards the place where the bladder was invaginated. It 

 bears its suckers exposed on its outer surface. In position and atti- 

 tude it thus resembles what we have occasionally seen, though only 

 exceptionally, in the older bladder-worms (Fig. 201). Like the head 

 of the future tape- worm, that of Cysticercus arionis seems to be a solid 

 body, which fills up by far the greater portion of the caudal bladder, 

 with which it is not, however, connected directly, but only by means 

 of a cylindrical or sac-like connecting part the future neck. 



From the analogy of the Cysticerci just referred to, I think we may 

 conclude that the head of Cysticercus arionis does not from the first appear 

 in this attitude, but that it only assumes it in consequence of a change 

 in the disposition of its parts. Here also I believe the development of 

 the future tape-worm begins with the formation of a hollow bud, 

 which starts from the wall of the enlarged embryonic body and grows 

 into the loose interstitial tissue of the same. This does not become 

 the sac-like and invaginated future neck portion in which the head is 



