160 M. Steenstrup on the genera Pachybdella and Peltogaster. 



in excellent observations, we find a representation of a very 

 remarkable irregular mass (fig. 19), which was quite full of more 

 or less developed ova. It was found in a crab, attached by one 

 end to the inner wall of the stomach, and with the opposite ex- 

 tremity somewhat squeezed in between two of the partitions, which 

 indicate the limits of the lateral parts of the original rings of 

 which the carapace is composed. In fig. 18 m w, Cavolini has 

 represented the ova contained in this mass in various degrees of 

 development, and in fig. 18 r r, two embryos just after their 

 exclusion from the egg. Cavolini compares these embryos with 

 the Onisci squilliformes described by Pallas, and confers upon 

 them this name. We cannot help seeing that the embryos thus 

 described and figured, are so very closely allied to Rathke's 

 Liriope, that they could not be distinguished without difficulty, 

 and we are consequently led involuntarily to compare them with 

 the larvae of Bopyrus. The form of the young larvae, therefore, 

 shows, that this irregular, ovigerous mass is in all probability 

 not only a transmuted parasitic Crustacean, but that it also 

 belongs to the family of the Bopyridse, only it is more shapeless, 

 or, as we might say, more monstrous than any other developed 

 form of that family, more even than the Peltogastri and Pachy- 

 hdell(B, and consequently this parasite is something more than an 

 Epizoon; for it was attached to an internal organ, like an 

 Entozoonj or intestinal worm, and especially like the extraor- 

 dinary mollusk Entoconcha mirabilisj discovered by J. Miiller in 

 Synapta digitata^. 



We have now got the following facts together : — The Bopy- 

 ridse are known only as parasites upon the higher Crustacea, — 

 the less irregular species of the genus Bopyrus occurring under 

 the carapace of the long-tailed Crustacea {Macrura), and the 

 most irregular, with which Rathke has even formed a separate 

 genus, under the abdomen of the same animals. The latter 

 forms consequently agree essentially, both in their residence and 

 external conditions of life, with Peltogaster and Pachybdella, which, 

 as we have seen, live under the abdomen of Paguri and Crabs 

 (Brachyura). Moreover, they approach these two parasites to a 

 certain extent even in their form. Thus they diff'er from the 

 more regular species of the genus in having the limbs, with the 

 exception of the anterior pair, completely lost on one side of the 

 animal; and the cavity for the reception of the eggs, which 

 occurs so universally amongst the Isopoda, instead of being 

 formed of several equally developed plates, is here composed 

 principally of a single plate, which has been developed at the 



* See Annals, 2nd Series, vol. ix. pp. 22 and 103, Jan. and Feb. 1852. 



