II4 THE CESTOIDEA 



passes through one pair of booklets, which is always carried hind- 

 most. The fate of this onchosphere is unknown, as feeding ex- 

 periments with appropriate hosts have been unsuccessful. But if 

 we may judge from other histories, this embryo is swallowed by 

 some invertebrate or perhaps a small fish on which the pike preys, 

 for the tissues of this fish sometimes contain numerous encysted 

 young forms of Bothriocepludus wormlike, with an invaginable 

 "scolex" or head at one end (Fig. XIL). Such a sexless, encysted 

 stage is known as a " metacestode " or " plerocestoid " (Braun). It 

 is by eating such infested fish that man 

 becomes the final host, in the Baltic pro- 

 vinces and elsewhere, where the pike is 

 a favourite diet, and is eaten in an im- 

 perfectly cooked condition. 



We have, however, more definite in- 

 formation about the history of Ligula 

 and Schistocephalus ; the ciliated larva of 

 FIG. xn. these worms is swallowed by a fish, and 



Metacestode of Bothriocephaius the six-hooked embryo makes its way 

 S). in l r through the intestinal wall, by the action 



of its booklets, and thus reaches the 



body cavity. Here it develops directly, by growth and loss 

 of booklets, into the strobila, and in the case of Schistocephalus, 

 the body even becomes segmented ; indeed, the parasite only differs 

 from the adult condition in the imperfect development of the 

 genital organs. The great increase in size of the worm causes con- 

 siderable inconvenience to the fish. When the latter is devoured 

 by a bird, the tapeworm soon becomes mature in the intestine, 

 the warmth of the bird's body hastening the development of the 

 genitals. According to Leuckart, the strobila, after two and a 

 half days' sojourn in the final host, may leave it through the anus, 

 partially digested, it is true, and the eggs are thus scattered in the 

 water. 



The meaning of the ciliated embryophore is variously inter- 

 preted as either (a) a primitive ectoderm, or (b) the remains of a 

 miracidium-like larva. The latter view commends itself at the pre- 

 sent day, and the free-swimming larva may be compared to that of 

 a Trematode, in which the " six-hooked embryo " may be regarded 

 as having developed simultaneously with its envelope (cf. Gyro- 

 dactylus, p. 53). 



CLASSIFICATION OF THE CESTOIDEA MKROZOA. 



In the Merozoa the generative organs, more especially the 

 uterus, exhibit two well-marked types of structure. In the one 

 type, as in the Monozoa and Bothriocephalus, the uterus retains its 



