73 



which is represented in Fig. 14, e, which will be observed to differ 

 very much from the more fully developed ones shown in Fig. 14, a, 

 a, a, and principally in the absence of the attenuated caudal extre- 

 mity. There are also sometimes to be observed in the pulpy sub- 

 stance of the parietes of the worm, elongated, oval bodies, of a granu- 

 lar structure, and firmly attached to the membrane, and which may 

 perhaps represent a still earlier stage of the embryo's growth, but 

 this is as yet very uncertain (Fig. 12). In the instance of the worm 

 now on the table, and which may be considered as approaching the 

 term of its latent condition, the young already correspond with the 

 description given above, and appear to be alike in all portions of the 

 worm. 



Having thus briefly described the chief points connected with the 

 anatomy of the worm, I will proceed with a few observations as to its 

 presumable nature. The now rather numerous facts that have been 

 collected with reference to the occurrence of an alternation of gene- 

 rations in many of the lower classes of animals, or it may almost be 

 said, in all the Invertebrata, in some mode or another, already afford 

 considerable ground for the support of an hypothesis which will 

 serve to throw considerable light upon the hitherto obscure relations 

 that obtain in many of the multiform creatures thrown together under 

 the name of Entozoa. 



In a recent work by a Danish naturalist, Steenstrup, published by 

 the Ray Society, a tolerably complete exposition will be found of all 

 that is yet known on this interesting subject. The longest and most 

 complete chapter of that work refers to the Entozoa, but principally 

 to the Trematode class, to which the author's own observations were 

 especially directed. The development of distomata of more than one 

 species is there, satisfactorily to my mind, deduced from Cercaria, 

 which live in water external to the animals which they afterwards in- 

 fest. The mode in which these parasites penetrate the water-snails 

 is clearly detailed, and reference is made to other observations tend- 

 ing to prove, that a penetration from without in earlier stages of 

 development of the parasites is not confined to those soft-skinned 

 animals alone, but takes place in fish, and even through the intesti- 

 nal canal of Batrachians. Having thus proved the penetration from 

 without and the existence of alternate generations, presenting entirely 

 different forms, in the Trematode class, the probable existence of a 

 similar alternation of generations in the Nematoidea is alluded to, 

 and the instance of one {SphcBrularia), a parasite of Hymenopterous 

 insects, is adduced as an instance of the kind. Though the supposi- 



