RHIZOCEPHALA 345 



crab has regenerated a hermaphrodite gonad. Parasitic castration is 

 the most evident expression of a remarkable and at present ill- 

 understood interference by the parasite with the general metabolism 

 of its host. 



Thompsonia (Fig. 251), parasitic on crabs, hermit crabs, etc., is 

 an extraordinary case of extreme reduction by parasitism, in which 

 an arthropod is degraded to the level of a fungus. The rootlets of the 



d.s. 



A B 



Fig. 248 . Stages in the development of Sacculina upon the mid gut of a crab. 

 From G. Smith. A, Early stage. B, Later stage, b, swelling caused by the 

 body of the Sacculina; c.t. central tumour upon which the body arises; 

 d.i., d.s. inferior (posterior) and superior (anterior) diverticula of the gut of the 

 host ; w. " nucleus " or rudiment of the body of the Sacculina ; op. opening of a 

 cavity in the central tumour, the "perisomatic cavity", from which the 

 definitive body eventually protrudes (not the mantle opening); rt. roots; 

 X. final position of the parasite. 



parasite are widely diffused through the host. Their branches in the 

 limbs give off sacs which become external at a moult of the host. 

 These sacs contain neither ganglion, generative ducts, nor testes, but 

 only a number of ova in a space of doubtful nature. When they are 

 ripe the ova have become (probably by parthenogenesis) Cypris 

 larvae, which are set free by the formation of an opening. There is no 

 parasitic castration of the host. 



