156 M. Steenstrup on the genera Pachybdella and Peltogaster. 



investigations. He has traced the development of the eggs 

 which fill these sacs in such enormous quantities, and ascer- 

 tained that the young proceeding from the sacs from both spe- 

 cies of crabs are of the same kind, and that the young is a true 

 Crustaceous animal. He finds a resemblance between these 

 young animals and the Cancer paludosus of Miiller (Zool. Da- 

 nica, tab. 48), but at the same thinks that they must be ap- 

 proximated to the Monoculus Telemus of Linnaeus*. On tab. 2. 

 fig. 15, Cavolini figures a portion of the eggs taken from ^'the 

 sac" that is the Pachybdella, under the abdomen of the crab 

 which he calls Ca7icer verrucosus; they are still immature, and 

 are united by mucous threads into chains. Fig. 16 «. represents 

 the mature egg with the developed embryo within it, and 

 fig. 16 c, the embryo just after its exclusion from the q^^, with 

 three distinct pairs of swimming feet, provided with bristles. It 

 cannot be denied that this embryo is exceedingly like that of 

 many Entomostraca, and we might therefore be induced at first 

 to regard the Pachybdella as a kind of Lernseid animal. But as 

 I must leave it to future observers, who may have the opportu- 

 nity of collecting these animals in large quantities, to give a 

 conclusive answer to this question, and only propose in the fol- 

 lowing pages to indicate some remarkable relations between the 

 Pachybdella and other forms of Crustacea, I will only add here, 

 that Cavolini, notwithstanding the experiments w^hich he made 

 with this view, did not succeed in tracing the further changes of 

 this embryo, and that he, in accordance with the spirit of his 

 time, did not suppose that the young animal itself might be- 

 come converted into the sac in which the eggs were found, 

 but rather that it was developed into a Crustacean, which after- 

 wards fastened this ovisac under the abdomen of the crabs. 



From Cavolini's observations, however, it appears that Pachy- 

 bdella is undoubtedly a Crustacean, and one which is gradually 

 brought into its sac-like form in consequence of its parasitic 

 mode of life. 



Even if we admit Diesing's separation of the original contents 

 of Rathke^s genus Peltogaster, — according to which the shorter 

 and broader form with the sucking pit at one end of the body, 

 which occurs under the abdomen of crabs, will form a peculiar 

 genus under the name of Pachybdella, and this we may be so much 

 the more inclined to do, as it appears from the preceding state- 

 ments that there are several species exceedingly closely allied to 

 it in external appearance, — we should certainly establish no 



* The Monoculus Telemus, Linn., which Linnseus himself says is " ge- 

 neris etiamnum dubii," is very clearly from his diagnosis and description 

 the Hyalcea tridentata of Forskal. 



