METAMORPHOSIS OF THE EMBRYO. 



727 



some partly projecting out of it, and others free in the body-cavity, 

 from which he concludes that it is possible that they reach the in- 

 testinal canal along with the first intermediate host, and that, after 

 being liberated by the digestive process, bore through the intestinal 

 wall and wander further. 1 



This being so, we are left to mere conjectures in regard to the 

 changes which the six-hooked embryo undergoes in assuming its 

 subsequent larval form. It seems probable, as we have already noted 

 (p. 377), that the embryo, after migrating into its host, increases in 

 size, loses its hooks, draws in one of the poles in order to form the 

 head, and by continuous longitudinal extension grows into the sub- 

 sequent larva. If the larval life of the worm be really divided between 

 two intermediate hosts, these modifications will probably take place 

 in the first, so that the second will only affect the parasite in so far 

 as it affords the possibility of further growth. 



If our views regarding the metamorphosis of the embryo of 

 Bothriocephalus be correct, the larva which is formed from the latter 

 represents a stage which we are quite justified in describing as 

 " cystic." It is true that the bladder and head-rudiment exhibit 

 very striking differences in form and structure when compared with 

 those of others, say of the bladder-worm of the pig ; but this need not 

 at all affect our conclusion, since the nearest relatives of the true 

 bladder-worms often present similar variations. We need only com- 

 pare with these worms the above described Piestocystis, to see in their 

 proper light the relations which exist between them. In both we 

 find a solid, somewhat elongated, worm-like body, whose develop- 

 ment from the six-hooked embryo can hardly be doubted, and in both 



FIG. 389. Young 

 bladder-worm from the 

 pike, with invaginated 

 head. ( x 6.) 



FIG. 390. Head of bladder-worm from the 

 pike. A, In longitudinal section; -B, In cross 

 section. ( x 40.) 



a retracted head without any sharp demarcation from the rest of the 

 body. The fact that this head is still less individualised than in 



1 Loc. cit., p. 50. 



