CESTOIDEA. 11 



embryo during its transformation into the scolex ; 

 and we are therefore ignorant whether the latter is 

 produced by metamorphosis or by gemmation, or 

 whether there are not several generations interposed 

 between the six-hooked vesicle and the scolex. 



Some, as yet incomplete, observations lead to 

 the idea that the embryo, when it has arrived at 

 its habitat, loses its hooks, and is developed into a 

 vesicle which produces the scolex by gemmation ; in 

 this case, the embryo would be a grand-nurse (Steen- 

 strup), or a proscolex (Van Beneden). But, if the 

 echinococcus be compared with the coenurus, it will 

 be understood that there is probably, in this respect, 

 no uniformity of development amongst all the 

 Taeniae ; and there are also probably many species 

 which do not pass through a vesicular form. 



The various phases of the development of a 

 toenioid worm are accompHshed in different situations, 

 as has been already observed. The adult individual, 

 the proglottis, is developed and hves exclusively in 

 the intestines ; the ovum is always expelled from the 

 intestines, and the embryo which it encloses must 

 undoubtedly, before it becomes adapted to hve in the 

 intestines, pass into a new stage of development 

 which brings it to the condition of a scolex, and 

 which is accompHshed in a different situation. The 

 hooks with which the embryo is armed, and which 

 are so arranged as to facilitate its movements through 

 a resisting, and not through a fluid, medium, would 

 support the supposition that this medium is a tissue, 

 or a dense parenchymatous structure, and this sup- 

 position is, to a certain extent, confirmed by the fact 

 of the constant absence of the larvae of the cestoid 



