252 THE THEORY OF THE GENE 



gaster or Sacculina, they then develop external charac- 

 ters like those of a female. In Fig. 142a, an adult male 

 crab is shown, with its large claws, and in a' the under 

 side of its abdomen with the copulatory appendages, and 

 in b an adult female is shown, with her small claws, and 

 in b' the under side of her abdomen with the setose bifur- 

 cated egg-carrying appendages. In c is shown a male 

 that has been infected at an early stage; the claws are 

 small, resembling those of the female, and the abdomen is 

 broad and female-like ; in c' the under side of the abdo- 

 men of the infected male is shown. It has small bifurcated 

 appendages like those of the female. 



The parasite sends long root-like processes into the 

 body of the crab, on which the parasite lives by absorbing 

 the juices, and may, in turn, set up physiological proc- 

 esses in the crab itself. The testes of the crab may not 

 at first be affected, but later may degenerate. In one case, 

 at least, where the parasite had fallen off, Geoffrey Smith 

 found large germ-cells developing in the regenerating 

 testis, which he interpreted as eggs. 



Giard left open the possibility as to whether the change 

 in the crab was due to the absorbtion of the testis, or 

 whether the action was more direct on the host. Geoffrey 

 Smith has brought forward some evidence relating to 

 fat in the blood, and certain arguments in favor of the 

 view that the change in the crab is due to the physiologi- 

 cal effects on the host. There is no evidence in Crustacea 

 that the destruction of the gonads affects the secondary 

 sexual characters. 



In insects, where there is evidence from castration, it 

 has been shown that the removal of the testes or ovary 

 does not alter the secondary sexual characters. It is all 

 the more significant, therefore, that in one case described 

 by Kornhauser, in one of the bugs (Thelia) that is para- 

 sitized by a hymenopter (Aphelopus), the male shows 



