Qg The Sexual Organization of the Rhizocephala. 



perfect agreement. In the whole group of Epicarida, as brought out by Bonnier (12), and as I have 

 confirmed for the Cryptoniscinae see Chapter 6 all the individuals are at first larval males which, 

 when young, crawl upon the adult parasites and fertilize them, and then fix themselves to 

 their hosts and themselves develope into females. All the individuals are, in fact, protandric 

 hermaphrodites. 



Applying the same idea to the Rhizocephala, we regard the degenerate Cypris males as 

 of exactly the same nature as the Cypris larvae which develope into the adult hermaphrodites, 

 but owing to the position which they have taken up they are arrested in development. Their 

 presence in this position is a reminiscence of a state through which the Rhizocephala have 

 passed, exactly similar to the Epicarida, in which larval males (being potential hermaphrodites 

 fertilized adult females (also potential hermaphrodites), but owing to the acquisition of self- 

 fertilization and the simultaneous ripening of the male and female sexual products, their instinct 

 for fixing, while still larvae, on other hermaphrodites is rendered useless. But we can under- 

 stand in some sense why this instinct has not been eliminated, because all the larvae, being 

 essentially males in derivation, must have possessed it, and the damage which its retention 

 does to the species is probably small, as only comparatively few larvae out of the myriads 

 produced ever find their way to the mantle-opening of a young parasite, instead of fixing on 

 a crab and infecting it. If we suppose however that the Cypris larvae fixed round the mantle- 

 opening really represent individuals of a true male sex quite separate from the hermaphrodite 

 individuals, then we must also suppose that a great number of larvae, probably about half, 

 are produced in every generation which are from the first doomed to uselessness, whether they 

 find young parasites to fix themselves onto or not, and this would be a solecism in the whole 

 field of nature. 



There is also an additional factor which may help to explain the retention of this 

 masculine instinct, and that is the essentially gregarious nature of most Cirripedes, which we 

 also see exhibited in several forms of Rhizocephala. It seems to me highly probable that this 

 gregarious instinct has been derived from the originally masculine instinct which led the Cypris 

 larvae to fix themselves upon an adult member of their own species whether female or 

 hermaphrodite, and was subsequently slightly modified so as to lead the Cypris larvae to fix 

 near other individuals instead of on them. (We may note in passing that Cirripede colonies 

 are built up by larvae fixing on an object which already holds specimens at various stages 

 of growth, and not merely by the simultaneous fixation of a number of larvae on the same 

 spot. The larvae also show a preference for fixing on the peduncle of the adult individuals.) 



The essential instinct, then, has always remained the same, and results in the occasional 

 fixation of a Cypris actually in its orginal position, adapted for fertilizing the individual on 

 which it is fixed, and under these circumstances it leads to arrested development and the 

 production of complemental males. 



To recapitulate — the complemental males in Cirripedes are in reality potential herma- 

 phrodites like the other individuals, which have been arrested in their development owing to 



