lOO CRUSTACEA 



to possess a root-system, but is attached to its host by a tube 

 which passes right through the mesentery and opens into the 

 ]aantle-cavity of the parasite. It may be suggested that this 

 tube corresponds to the stalk of the normal Cirripede, but its 

 exact mode of formation would certainly throw much light on 

 the question of Ehizocephalan development. 



Phenomena of Growth and Sex in the Crustacea. 



In the foregoing account of the Girripedia we have met with 

 certain peculiar sexual relations in which closely allied species 

 exhibit marked differences in regard to the distribution of the 

 qualities of sex among their individuals ; we have seen that the 

 majority of species are hermaphrodite, unlike most Crustacea 

 which, with the other exception of the parasitic Isopoda, are 

 normally dioecious ; and that in some species complemental 

 males exist side by side with the hermaphrodites, while, in yet 

 others, the individuals are either females or dwarf males. 



Before examining the causes of these conditions, it will be 

 opportune to consider a number of facts which throw light on 

 the question of sex and hermaphroditism in general. We may 

 then return to the discussion of the hermaphroditism found in 

 particular in the Girripedia and Isopoda. 



Parasitic Castration. — Giard -^ was the first to observe that 

 a number of parasites exert a remarkable influence on the sexual 

 characters of their host, such that the generative glands become 

 reduced, or may completely degenerate, while the secondary 

 sexual characters become materially altered. This was proved 

 to occur in the most widely different hosts, affected by the 

 most widely different parasites {e.g. Grustacea, Insecta, Worms). 

 Moreover, it was apparent that the affection does not consist 

 in the parasite merely destroying the generative organs, 

 with which it often does not come into contact, but rather in the 

 general disturbance of the metabolism set up by its presence. 



The most completely studied cases of parasitic castration are 

 those of the Ehizocephalous Sacculina neglecta, parasitic on the 

 spider-crab, Inachios mauritanicus^ and of Peltogaster mcrvahis 



1 Bull ,S'c. D^p. Nord (2), 10 Ann. xviii., 1887, p. 1. Ihid. (3), i., 1888, p. 12 ; 

 and other papers. 



^ G. Smith, loc. cit. chap. v. 1. scorpio should bo /. mauritaniciis throughout 

 tliis Monograph. 



