Cunningham, Unisexual Inheritance. 39 



male crabs which are attacked by Sacculina the unisexual modifications 

 have begun to appear, and that in subsequent development there is a 

 regressive development of them. Another doubtful point is the history 

 of both male and female crabs after the Sacculina has died. For the 

 parasite does not kill the crab, it merely renders it incapable of re- 

 production. According- to Del age the Sacculina lives a little more 

 tha nthree years, and then dies a natural death, after having produ- 

 ced its eggs. After the death of the parasite the crab may recommence 

 to moult and to grow, but the investigator does not say whether the 

 crab recovers its fertility, nor has Giard discovered whether the sup- 

 pressed secondary sexual characters of the male are recovered after 

 the death of the parasite. Delage says he once met with a large 

 crab which had no external Sacculina, or any trace of the scar where 

 the peduncle had been attached, but around the intestine internally 

 there were roots of a Sacculina in process of degeneration: he does 

 not describe the sexual characters of this crab. Evidence there- 

 fore of the history of the crab after the death of the parasite is still 

 to be obtained. 



Other modifications of secondary sexual characters occur in both 

 male and female shore crabs infested by Sacculina: in the normal 

 male the first and second segments of the abdomen alone bear 

 appendages which are modified as copulatory organs into stiff stylets: 

 in the infested specimens these are more or less reduced in size. In 

 the normal female the segments 2 to 5 bear four pairs of long appen- 

 dages with two branches to which the eggs when laid are attached: 

 these are reduced in size in the infested females. 



\MStenorhynchus phalangium the sexual dimorphism is more 

 pronounced than in Carcinus maenas, the difference between the 

 sizes of the first pair of legs, the pinch er claws, in the males and 

 females being much greater. In male Stenorhynchus infested with 

 Sacculina the pinch er claws are no larger than in the female. In 

 another form, Inachus Scorpio, the zoologist Fraisse was decei- 

 ved by the effect of the parasite on the special characters of the males, 

 and mistook them for females, so that he came to the conclusion that 

 the males were never infested. 



Another crustacean parasite belonging to a different order, namely, 

 thelsopoda, lives in the branchial cavity of prawns and although it 

 sends no processes into the interior of its host, but is entirely an 

 external parasite, it nevertheless causes the host usually to be sterile. 

 Giard does not say whether this is due to any special effect on the 

 generative organs, but it appears to be due merely to the general 

 effect on the mutrition of the host. However this may be, Giard 

 has shown that the secondary sexual characters of the host are here 

 also more or less completely suppressed. So much is this the case 



