§Q The effect of the Parasites on their Hosts. 



the host would be greatly diminished, and so a greater number of Entoniscids would survive 

 when associated with a Sacculina, than in crabs nninfected by Sacculina. 



Whatever the explanation may be, the association of the two parasites is of great 

 biological interest. 



B. The nature of the parasitic castration in Eriphia spinifrons. 



It is shown by Dr. Duncker's notes that Sacculina has no obvious effect 1 ) on the secon- 

 dary sexual characters of Eriphia spinifrons but that a very perfect effect is produced by a 

 parasite belonging to the family of the Entoniscidae. This effect differs from that described 

 for Inachus scorpio in that the modified crabs ali belong to a homogeneous and sharply defined 

 category which shows no gradation into the female or male condition. One partial exception 

 to this mie was observed however, namely in a modified crab which carried a single rudi- 

 mentary female appendage on one side of the abdomen. 



We are therefore confronted with the same problem which occurred to us in the case 

 of the perfect external hermaphrodites of Inachus scorpio, namely to determine to what sex the 

 modified crabs originally belonged. A perfectly definite and irrefragable answer is given to 

 this question in the next case to be examined, Eachygrapsus marmorattis, but in the present 

 case the statistics strongly suggest to us that ali the hermaphrodite forms are in reality 

 modified males, as we found to be probably the case in Inachus scorpio. For the statistics 

 show that unless we add the hermaphrodite forms infected by Sacculina to the undoubted 

 males so infected, there is an extraordinary preponderance of infected females over infected 

 males, i. e. more than twice as many females are infected than males, in proportion to the 

 whole number of the two sexes found in nature. But if we add the infected hermaphrodites 

 to the infected males the proportion of infected males and females is made about equal, and 

 this is what we should expect from our knowledge of the method of infection of the parasite. 



Although this argument is not conclusive, it is perhaps a suggestive prelude to the 

 definite proof which is furnished in the next case to be considered. 



4. The Case of Pachygrapsus marmoratus. 



This crab in the Bay of Naples is very commonly infested by a species of Sacculina 

 named by Kossmann S. benedeni: it also harbours a species of Entoniscid called Grapsion 

 Cavolinii (Giard). Of some 200 specimens examined by me only five contained Grapsion Ca- 

 volinii, while about 70 had Sacculina either externally or internally. The five specimens which 

 harboured Grapsion were also infested by Sacculina so that it is highly probable that the re- 

 markable coincidence of these two parasites which was discovered in Eriphia spinifrons also 



*) Subsequent researches have shown that Sacculina may also exert some infhience in this species without the 

 cooperation of Entoniscus. 



