384 THE INVERTEBRATA 



new cuticle. The male crabs have a much broader abdomen, reduced 

 copulatory styles (these may disappear altogether), and abdominal 

 svvimmerets (which carry the eggs in the female, and are absent in 

 the normal male). There is, in short, a marked tendency to the female 

 type. In the female crabs there is also a change, but this is held to be 

 not towards the male but towards the juvenile type. The gonads dis- 

 appear, but cases have been observed in which the parasite has been 

 killed and months afterwards what was probably an originally male 

 crab has regenerated a hermaphrodite gonad. Parasitic castration is 



d.i — 



A B 



Fig. 259. Stages in the development of Sacculina upon the mid gut of a crab. 

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

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

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

 host; n. "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. 



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

 stood interference by the parasite with the general metabolism of its 

 host. 



Thompsonia (Fig. 262), 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 

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



