gg The effect of the Parasites on their Hosts. 



The discovery of the phenomenon of parasitic castration is due to Giard who in a series 

 of valuable papers (see Bibliography p. 93} has shown that the sexual condition of a number 

 of hosts is profoundly modified by the presence of certain parasites. 



It appeared to me that by the detailed examination, both by statistics and dissection, 

 of a few types, a more exact knowledge of this question could be obtained, and for this pur- 

 pose I have chosen to study especially the effect of Sacculina neglecta on Inachus Scorpio, be- 

 cause both host and parasite are very common at Naples, and the sexual dimorphism of 

 I. scorpio is very highly marked so that any modification is easily detected. The following 

 remarks therefore mostly apply to this particular case, and it will be seen that many new 

 facts of importance have come to light, especially with regard to the internal condition of 

 infected individuals. After considering in detail the case of Inachus scorpio, some further facts 

 are added in reference to Pachi/qrapsus marmoratus and Eriphia spinifrotis. 



1. General effect of S. neglecta on I. scorpio. 



AYhile the parasite is internal it is difficult to calculate what effect it has on the 

 growth of its host, but it is probable that when the system of roots is well established growth 

 is considerably retarded. This supposition is confirmed by the fact that Sacculinae internae 

 are more frequently found in crabs which have not moulted for some time. The parasite 

 becomes external invariably immediately after, and as the result of, a moult on the part of 

 its host, and from this point onward until the death of the parasite the infected crab never 

 moults again. Infected crabs are more sluggish in their movements than uninfected specimens 

 and they cannot resist so well the irregularities of aquarium life. After the death of the 

 parasite the crab may recover, but the process is slow, and exceedingly few recovered crabs 

 are met with in nature, chiefly because the life of /. scorpio is probably never much prolonged 

 over two years and in consequence crabs that have passed through the disease have mostly 

 fulfilled their term of existence. And yet the rarity of recovered crabs must in some measure 

 be put down to the fatality of the disease, since a great many young crabs especially in 

 autumn have external parasites and yet few survive into the succeeding summer. 



2. Effect on the primary and secondary sexual characters. 



It is in this respect that the most interesting effect is to be observed. In the treatment 

 of this highly important subject I propose to divide my data under separate headings, exhibiting 

 first A) the effect exerted by the parasite on the secondary sexual characters: in order to do 

 this the rate of growth and the development of the various characters in infected males and 



