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Art. X\A\.— Natural History and Metamorphosis of an 

 Anomalous Crustaceous Parasite of Carcinus Mcenas, the 

 Sacculina Carcini. By J. V. Thompson, F.L.S. Deputy- 

 Inspector-General of Hospitals. 



A long time previous to the discovery of the metamorphosis 

 in the Crustacea, I had occasionally met with the common 

 shore-crab {Carcinus Mcenas), having a purse-like appendage 

 attached to the under-side of the tail, (figs. 1 and 2.) The 



Fig. 1. 



Fig. 2. 



first of these being a female, it became a question whether 

 this crab might not differ from others known to carry their 

 ova after exclusion, attached in naked groups to the false feet 

 under the tail. This was speedily decided, by finding males 

 with the same appendage ; and by individuals of both sexes 

 being occasionally met with, having two or even three of them, 

 but always attached to the median line of the tail, and to the 

 interstices of some of its uppermost joints. These anomalies 

 appeared to declare that they could be no part of the crab 

 itself, but some anomalous excrescence or parasite. 



These parasites, which may be seen of various size, resemble 

 a leathern pouch or satchel in figure and texture, are perfectly 

 symmetrical, having an opening drawn together and closed at 

 the lower end, and are so attached by a short thick neck to the 

 membranous interstice between one of the upper joints of the 

 tail of the crab, as to appear continuous with the body of the 

 animal. On removing them by force, the neck presents the 

 appearance of irregular branched joints, and a large opening 

 is seen, which has every appearance of being continuous 

 between the rectum of the crab and the cavity of the parasite, 

 so as to conduct us to the conclusion, that it is through_ this 



