154 M. Steenstrup on the genera Pachybdella and Peltogaster. 



place the genus Pachyhdella amongst the Vermes, accepting this 

 class in its widest sense. In this class, however, Rathke has given 

 his genus no determinate place ; he only thinks that these ani- 

 mals are by no means to be arranged with the Bdellide or Trema- 

 tode worms, of which we are at once reminded by the form of 

 their bodies, the pits with which they attach themselves, and their 

 parasitic mode of life ; he would rather regard them as transition 

 forms from the Worms to the Actinia and the Radiated animals 

 most nearly allied to these. Thus, according to his view, the 

 pit with which these sac-like parasites attach themselves to the 

 abdomen of the crab, is only a sucker, from which no tube leads 

 into the body of the animal ; and the other opening, which exists 

 at the free extremity of the body, is a mouth, which leads into a 

 large cavity, serving at once as a digestive cavity and for the 

 development of the ova, — a double office which he considers to 

 be proved by the extended observations which he was enabled to 

 make upon the P. Pagwi, but which we must admit with the 

 author is " something very peculiar and hitherto unheard of in 

 the case of a worm." 



Our knowledge of Diesing's so-called genus Pachyhdella is 

 however fortunately not confined to the scanty information, 

 welcome as it may be in every respect, which Rathke has fur- 

 nished upon the individuals found and examined by him ; other 

 statements are extant, which, although they have hitherto been 

 overlooked, are deserving of every attention, as they throw a 

 light upon the distribution and systematic position of this 

 parasite. 



With regard to the occurrence and diffusion oi Pachybdella, I 

 will refer to the sac-like parasite found by Bell under the abdo- 

 men of Carcinus Manas and Portunus marmoreus from the Bri- 

 tish Channel, and which he has described in such a manner, 

 that there can be no doubt he had a Pachyhdella before him. 

 On the first-mentioned crab he appears to have found the parasite 

 only occasionally, but it occurred in considerable numbers on 

 the other. This author, who only refers to the parasite en pass- 

 ant, regards the pit by which it attaches itself as the oral aper- 

 ture, and the other orifice, which Rathke considers to be the 

 mouth, as the anus. Bell consequently ascribes a complete di- 

 gestive canal to the animal. The parasites which occurred upon 

 Carcinus Manas w^re so similar to those which Bell found in 

 greater abundance upon Portunus marmoreus, that he docs not 

 appear to have felt any doubt as to the identity of the species. 



I can prove the occurrence of Pachyljdclla not only in the 

 Chainiel, but also in the Mediterranean, from several individuals 

 in the Zoological Museum of this University (Copenhagen). In 

 the year 18i8 I received a few Crustacea from the former locality, 



