in the Common Shore-crab caused by Sacculina. 97 



In the first of two papers on " La Castration Parasitaire " 

 Giard mentions the fact that a certain alteration in the character 

 of the abdomen can be noticed in male shore-crabs attacked by 

 Sacculina. The diagrams which he gives show plainly that the 

 infected males often possess an abdomen broader than that usually 

 associated with this sex, yet not attaining the full trough-like 

 development of the female. It was Giard's view that the female 

 might approach the male type when parasitised ; he does not here 

 however refer to any modification of the female. Though no 

 definite statement is made the reader of this paper is led to 

 assume that atrophy of the gonads takes place. 



To understand the account of the changes in the sexual 

 characters, certain particulars of the life history need to be supplied. 

 The elucidation of the extraordinary development of Sacculina 

 is found in a brilliant paper by Delage*. The fertilised eggs 

 develop into free swimming Nauplius larvae. These fix at the 

 base of hairs on the carapace of crabs, and the almost disorganised 

 mass of cells which has resulted from histolysis of the larval body 

 passes through the gap in the carapace which the articulation of 

 the hair affords. It wanders through the body cavity, and even- 

 tually becomes attached to the intestine in the region of the 

 abdomen. Here it grows, absorbing nutriment from the blood, 

 and attains to the development of the adult parasite. The in- 

 ternal stage of development is terminated by a moult. For if 

 the Sacculina is ripe for emergence it is found that the part of 

 the parasite destined to become external has eaten its way through 

 the muscles and epithelium of the abdominal wall of the host and 

 when the chitinous exoskeleton is cast, this visceral mass emerges. 

 Connection with the internal root-system is maintained by the 

 narrow peduncle occupying the aperture which served for escape. 



In crabs (Brachyura) the abdomen is clothed with a fairly 

 thick exoskeleton. It follows that when the new shell is formed 

 after the moult described above the Sacculina acts as a rivet and 

 prevents the fleshy part of the abdomen, in any subsequent 

 attempt at moulting, being withdrawn from its chitinous case. 

 The fact that a crab infected by Sacculina is prevented from 

 further moulting and consequently from growth after the parasite 

 becomes external is well known, but I consider it to be due to the 

 mechanical disability, which I have pointed out above, alone. It 

 might conceivably be a repression of growth occasioned by the 

 drain of nutriment for the parasite or the action of some specific 

 substance secreted by that organism. But if this were so we 

 might expect to find a tendency to inhibit the moult which intro- 

 duces the Sacculina to the external world, also at work. For 



* Arch. Zool. Exp., Ser. 2, Tome ii. 1884, p. 417. 



