98 DEVELOPMENT OF 



stance, represented the two divisions of his genus Di- 

 plostomum, and the third, that of Holostomum ; so that 

 his Biplostommn davatum is the larva, Holostomum cu- 

 ticola"^ the pupa, and Diplostomum volvens the full-grown 

 Trematode. 



Now since we see (1. c.) that Nordmann has observed 

 fifty-eight species of entozoa in the eyes of different fish, 

 which belong principally to the two genera named Holos- 

 tomum and Biplostommn, and that he has formed two 

 separate groups of the latter genus, the one claviform, 

 corresponding with D. clavatiim, and the other broader and 

 flatter, with D. volvens, I am certainly much inclined to 

 assume that the one group is the larva condition of the 

 other ; and Nordmann's own words appear to support me 

 in this supposition, since he was himself inclined to con- 

 sider them to be connected as younger and older forms, if 

 only the transition form between them had been discovered. 

 We cannot expect to meet with such if ^^pupa state forms 

 the intermediate link, and on this account we may pre- 

 liminarily regard the encysted Holostomum as a pupa. 

 According to the remark, p. 102, it appears that most, if 

 not all Holostomata in fishes are thus encysted, and con- 

 sequently that they w^^pupce. Now if we assume, as our 

 present knowledge would seem to justify us in doing, that 

 these forms (of Bistoma) are peculiar to the e^es of fish, 

 as they have not been observed in other organs, and if we 

 allow its proper weight to Nordmann's observation that 

 the pupce or encysted Holostomata are frequently found 

 over the whole skin of the fish, either immediately under 

 the epidermis, or imbedded in the most external layer of 

 muscle, we shall again perceive a ready explanation of the 

 fact that many of these animals must necessarily fail of 

 being developed even though they have reached the or- 

 ganism at the expense of which they are destined to live, 



* In Ids description, the Holostomum ctiticola is represented as of a larger 

 size than my pupce, but the form is the same. Holostomum hremcaudatum of 

 Nordmann, which was not found in a capsule, much resembles an individual 

 which has just quitted the pupa case, and remains in a contracted state ; its 

 size corresponds exactly with this. 



