726 OCCURRENCE AND DEVELOPMENT OF BOTHRIOCEPIIALUS IATUS. 



such cases the inner one remains for a time in connection with the 

 embryo in the form of a distinct delicate skin, presenting the appear- 

 ance of a clear albuminous layer (Fig. 388). It is difficult to deter- 

 mine how long the worm can continue in this free condition without 

 the mantle. Its appearance is for some time so thoroughly normal, 

 and its mobility so great and constant, that I cannot agree with Knoch, 

 who is inclined to regard the bursting of the mantle and the issue of 

 the embryo as signs of a miscarriage. On the contrary, I should con- 

 sider it probable that, as in the ciliated embryos of the Trematodes, 

 this event takes place when the animals enter their intermediate host. 

 If my opinion be correct, the ciliated coat is of use only in the search 

 for the latter. For the embryos do not enter the tissue of their host 

 from the intestine, but make their way directly into it from without, 

 and in so doing they are much aided by the powerful movements 

 of the hooks and of the whole body. 



This process has unfortunately hitherto quite escaped observa- 

 tion. All attempts to bring about an immigration of the embryos of 

 Bothriocephalus have failed. I have several times kept young pike 

 for a week in the same aquarium with ciliated embryos without 

 infection taking place. Just as unsuccessful were the experiments of 

 Schauinsland, who several times, by means of a pipette, introduced a 

 large number of ciliated embryos into the stomachs of young burbots, 

 but could not find that they had bored into the walls of the intestine, 

 or even pierced them. Twenty-four hours later they were found 

 usually still living, and for the most part still surrounded by their 

 cilia, chiefly among the pyloric caeca. 1 



These negative results are the more striking because it has been 

 certainly demonstrated that pike and burbot are the intermediate 

 hosts of Bothriocephalus. Whether the animals experimented upon 

 were already too old for infection, or whether the embryos migrate, 

 in the first place, into other aquatic animals, perhaps invertebrates, 

 which are subsequently eaten by the fish when their inmates have 

 attained a certain stage of development, are in the meantime open 

 questions, which can only be answered by further investigations. 

 The conditions of development in Trematodes, to which we have 

 already referred, seem to favour the latter supposition; and that 

 all the more since the larvae of Bothriocephalus in the pike 

 and burbot were never observed by Braun in the first stages of 

 development, and were always of a definite size (8 to 10 mm.). 

 Braun, who inclines to the same opinion, advances a number of 

 further observations in its favour. He finds that the larvae are 

 specially numerous in the wall of the intestine, and has observed 



1 Loc. cit., p. 26. 



