6. General discussion. 35 



separation of the sexes, how strange must it appear to find them in an ancient group of Cirri- 

 pedes in a degenerate condition! 



I am also unable to look upon the presence of complemental males in such a form 

 as Scalpellum vulgare, as a progressive differentiation of the sexes, because the hermaphrodite 

 individuals of Scalpellum mlgare are perfectly capable of, and do indeed effect, cross-fertilization, 

 and the particular use of the minute complemental males is very difficult to understand. I am 

 inclined to regard them far more as approaching the rudimentary condition actually found in 

 the Khizocephala. 



I would suggest that the explanation of the whole matter must be sought in the fun- 

 damental nature of secondary hermaphroditism, i. e. an hermaphroditism which has been imposed 

 on an originally dioecious organism as we must certainly suppose the original ancestors of the 

 Cirripedia were. 



In Chapter 5 of this book, in the light of which the present discussion must be read, 

 certain evidences drawn from the phenomena of parasitic castration are put forward to 

 show that in all cases of secondary hermaphroditism, the hermaphrodite state is im- 

 posed on the male organism and on the male organism alone, the female 

 being incapable of assuming this condition. I am therefore led to look upon the ordinary 

 hermaphrodite Cirripedes as simply representing the male sex, the female having been sup- 

 pressed, and the occurrence of complemental males in an arrested condition of development 

 as in Scalpellum, or in a degenerate state as in the Bhizocephala, is due to the peculiar 

 condition of protandric hermaphroditism through which the animals at some time have passed. 



According to my view, the most primitive condition is exhibited in such forms as 

 S. velutimim, I. quadrivalvis, and the Abdominalia, in which the male sex becomes mature while 

 still in a semi-embryonic condition and is affixed to the female. This male sex, in the subse- 

 quent evolution of the group, next became hermaphrodite, and instead of always fixing on the 

 females began to start an independent life, though the old habit of fixing on other individuals was 

 kept up by those larvae which happened to find either a female or an adult hermaphrodite on 

 which to fix. With the extinction of the whole female sex, whose use in propagation would be 

 precarious, the larvae which happened to find other individuals to fix on, would still do this, 

 but it would be not upon females but always upon adult protandric hermaphrodites, the larvae 

 themselves being potentially protandric hermaphrodites, but in the cases where they became 

 fixed to other individuals not developing beyond the male state. This I take to be the actual 

 case in Scalpellum vulgare etc., the so-called complemental males being in reality protandric 

 hermaphrodites like all the other individuals , but arrested in development owing to their 

 peculiar position on the hermaphrodites. Exceedingly suggestive in this connection is the fact 

 that in Scalpellum Peroni the complemental males do show a trace of incipient hermaphroditism 

 as remarked by Gruvel (15 p. 152). 



Now if we compare this idea with what actually happens in the only other group of 

 hermaphrodite Crustacea, namely the Epicarida or parasitic Isopoda, we find that it is in 



