THE GENERATIVE ORGANS. 



681 



With the persistence of the generative glands is further associated 

 the necessity of enveloping the ova, which are liberated very early 

 (even before the development of the embryo), in a resistant shell 

 (p. 321). The free mobility which is exhibited by the 

 great majority of embryos of Bothriocephalus may also 

 be so far associated with the early liberation of the ova, 

 since it replaces the movement of the proglottides, 

 increasing the chances of the developing brood reach- 

 ing its destination. 



The separated segments do not for long retain the 

 free and independent mobility which characterises the 

 proglottides, especially of the large-jointed Taeniadse. 

 This is evidently the necessary consequence of their 

 mode of separation which does not take place in 

 single segments, but in longer or shorter pieces, which 

 remain united. 1 



The adult Bothriocephalidse live either in cold- 

 blooded or warm-blooded hosts, but only in carnivorous 

 forms, and almost always in those which feed on water 

 animals, and especially on fishes. Besides predacious 

 fish, these parasites especially infest water birds and 

 seals. I say especially, for there are also several land 

 animals, mostly mammals, which under certain circum- 

 stances harbour Bothriocephalidse. ArcJiigetes is a 

 unique and conspicuous exception, both in its occur- 

 rence and in its whole life-history, since it attains 

 sexual maturity, as has been already more than once Archlgetes Sie- 

 mentioned, in the intermediate host, and that a worm. *>oUi. ( x 60.) 



The Distribution of the sexually mature worms suggests an 

 inference as to the occurrence of the young forms. The embryos 

 attain development especially in fishes, and more frequently in those 

 inhabiting rivers, than in marine forms. The other vertebrate groups 

 are not, however, wholly spared ; young Bothriocephalidse have been 

 found in frogs and reptiles, in birds and mammals (species of Spar- 

 ganum, Dies., and Ligula, Dies.), and even in animals which are 

 only temporarily associated with water. Even man is in this con- 

 nection, as we shall see, no exception. 



1 According to the observations of Eschricht and others, there are, indeed, in fishes 

 some Bothriocephali, which regularly throw off all their joints during summer, so that only 

 the heads remain, again producing during winter new segments, all attaining sexual 

 maturity. In such cases the liberation of ova is not continuous, but periodic. To these 

 belong, apparently, those species especially which produce naked embryos, developing 

 within the joints, from thin shelled ova, with but little included nutritive material (p. 327) ; 

 species, therefore, which in their mode of reproduction present considerable resemblance 

 to the Taeniada}. 



