M. Steenstrup on the genera Pachybdella and Peltogaster. 155 



from M. V. Prosch, a ship-surgeon, and amongst these was a 

 Portunus hirtellus, under the abdomen of which there was a very 

 large sac, which I took to be a Lernseid parasite, without how- 

 ever being able to refer it to any known form of the family of 

 Lernseidse. I have since, however, recognized in it a form ap- 

 proaching so closely to Pachybdella Carcini, that it could only 

 be distinguished therefrom by a few immaterial points in the 

 outline of the body, and the wrinkling of the hinder opening. 

 From a specimen of Carcinus Mcenas, taken by Captain Svenson 

 on the " Black banks ^' in the North Sea, I obtained a very large 

 individual of a form exactly agreeing with P. Carcini^ Rathke ; 

 and I have since found three other individuals, resembling the 

 first, in a bottle containing several specimens of Portunus hir- 

 tellus from the Mediterranean. 



If the preceding statements would lead to the supposition 

 that this form of parasite is by no means to be reckoned amongst 

 the greatest rarities of these seas, this appears still more di- 

 stinctly from some observations which I found in Cavolini's 

 memoir upon the Development of Fishes and Crustacea, on 

 reading it through last autumn with a very different object. 

 From this I found that Cavolini had not only known and figured 

 these animals, but that he had had them in quantity, and, as it 

 appears from his figures, several species of them, found under the 

 abdomen of two species of crabs, which are called Cancer de- 

 pressus, Fabr., and C. verrucosus, Forsk., in his memoir. These 

 sacs are found both upon the males and females ; they are repre- 

 sented under the abdomen of the males of both species (tab. 2. 

 figs. 1 & 14), and under that of the female of the former (fig. 13). 

 They occur so commonly, that the fishermen are generally of 

 opinion that the eggs of the crabs are contained in them. The 

 form represented under the tail of the C. verrucosus in Cavolini's 

 fig. ] 4, so closely resembles the P. Carcini figured by Rathke, 

 that it might be taken for the same species ; that represented in 

 figs. 1 & 13 on Cancer depressus rather resembles the one above 

 mentioned from Pagwus hirtellus, but Cavolini states that the 

 opening at the narrow end has only four knots or teeth, whilst 

 my specimens had at least twice that number (8-10). The com- 

 parison of the outline of the animal with that of the fruit of 

 Thlaspi Bursa-pastoris is very characteristic, but shows that his 

 specimens must have been somewhat longer in proportion than 

 those which 1 obtained from Pagurus hirtellus. 



From the above observations it is sufficiently evident, that 

 these Pachybdella-like parasites are not only more abundant 

 than has been supposed, but that several species of them occur, 

 according to the various species of crabs upon which they live. 

 But still more important consequences result from Cavolini^s 



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