336 Prof. A. Giaid on Parasitic Castration 



V. 



What conclusions can we draw from the observations 

 whicli we have just summarized? 



Keeping for the moment to the facts taken in themselves, 

 without attempting to inquire into their primary cause, and 

 considering them in their maximum state, that is to say, such 

 as we observe in Stenorhynchus phalangimn, we shall see at 

 once that the parasite, when fixed upon a male crab, is much 

 better protected than it would have been if the male had not 

 undergone the modification already described. As, however, 

 tins transformation does not take place unless the crab is 

 infested when quite young, at a time when the sexual differ- 

 entiation has not yet been produced, it follows that natural 

 selection must have determined a more and more precocious 

 fixation of the parasite *. Hence it happens that the Saccu- 

 lince of the Oxyrhyncha (1) always infest young crabs, and 

 (2) cause a more complete atrophy of the copulatory styles 

 and ovigerous feet. 



In the Brachyura of which the sexual dimorphism is less 

 accentuated the protection afforded to the parasite by the 

 modified abdomen of the male is less efficacious ; consequently 

 natural selection plays a less active part, and we pretty fre- 

 quently observe the infestation of old individuals. More- 

 over, even in case of early infestation, the modification of the 

 male sexual characters is less considerable. 



If we now seek to understand the mode in which the modi- 

 fications of the external sexual cliaracters of the Decapods 

 are connected with parasitic castration, several explanations 

 occur to the mind and must be examined in their turn. 



1. It may be supposed that these modifications are useful 

 to the infested crab, and, in consequence^ have been gradually 

 developed by natural selection. 



It may seem strange, a priori^ to invoke natural selection, 

 and consequently heredity, to explain phenomena which are 

 associated with the sterility of the animals which manifest 

 them ; but we must not forget that this sterility is only tem- 

 porary, that it ceases with the existence of the parasite, and 

 that it is not impossible that a crab which has borne a Sac- 



Tamias Lysteri, Rich., which, according to Asa Fitch, is often castrated 

 by an CEstrid larva, Cuterebra emasculator, Fitch, wbich resides in the 

 testicular sac. We would suggest to our American confreres the com- 

 plete investigation of this parasite and of the effects which it produces. 



* Of course at present we have to do only with natural selection as 

 applying to the parasite. 



