212 THE SEAS 



continue to swim about and then must either die or find a 

 crab on which to settle. In the latter case they attach 

 themselves by means of tiny hooks on their feelers to hairs 

 on the body of the crab, the feelers penetrating the base 

 of the hair after which the legs of the Sacculina are cast 

 off and the rest of the body degenerates into a little mass 

 which passes through the hollow feelers into the crab's 

 body. It is carried in the blood stream until it reaches the 

 middle of the body where it attaches itself near the stomach, 

 sending out roots in all directions, while the main mass grows 

 larger and larger until it gets pressed against the inside 

 of the shell about the middle of the under side of the body. 

 The next time the crab moults the parasite pushes its way 

 out before the new shell has had time to harden and there 

 assumes its final tumour-like shape. 



The parasite usually lives some three or four years 

 and from the time it escapes to the exterior the host 

 crab never moults, all its reserve of food and strength go 

 to supply the needs of the parasite. It is only when the 

 Sacculina dies that the crab is able to resume growth and 

 moult at the usual intervals. It is a very interesting fact 

 that male crabs which are infected in this way gradually 

 assume the appearance of females, developing a broad 

 instead of a thin abdomen, smaller claws, and certain 

 small legs on the hinder end of the body which are normally 

 only possessed by the females. About seventy per cent, of 

 infected male crabs show various stages in this process but 

 infected females on the other hand show no signs of becoming 

 like males. An elaborate theory of the causation of sex 

 has been based on this sex change in parasitized crabs. 

 Details of this are out of place here, but the gist of it is 

 that the presence of the parasite causes the active male 

 to lead the more sedentary life of the female, and to eat 

 more in order to counteract the abnormal drain on its 



