IO6 CRUSTACEA 



CHAP. 



of small ova in their generative glands, which, however, never 

 come to maturity. 



The condition of the degenerate males in the Rhizocephala 

 may also be interpreted in the same manner. These never pass 

 beyond the Cypris stage of development, in which they resemble 

 in detail the Cypris larvae of the ordinary hermaphrodite 

 individuals, and they are quite useless in the propagation of 

 their species. 



It is more reasonable to suppose that these Cypris larvae, 

 which fix on the mantle-openings of adult parasites, are in 

 reality identical with the ordinary Cypris which infest crabs and 

 develop into the hermaphrodites, than that they represent 

 a whole male sex doomed beforehand to uselessness and degenera- 

 tion. If we suppose that the Cirripedes have passed through 

 a state of protandric herrnaphroditism similar to that of the 

 Isopoda Epicarida, it is plain that all the larvae must have 

 originally possessed the instinct of first fixing on the adult 

 parasites, and we may suppose that this instinct has been retained 

 in the Ehizocephala, but is now only actually fulfilled by 

 a certain proportion of the larvae, which, under existing 

 circumstances, are useless and fail to develop further ; while the 

 rest of the larvae, not finding an adult parasite to fix upon, go 

 straight on to infect their hosts and develop into the adult 

 hermaphrodites. 



The same explanation would apply to the complemental 

 males in Scalpellum, etc., these individuals being also potential 

 hermaphrodites, which are arrested in development, though not 

 so completely as in the Ehizocephala, owing to the position they 

 have taken up. 



This theory throws light on another dark feature of Cirripede 

 life -history, namely, the gregarious instinct. The associations 

 of Cirripedes are not formed by a number of Cypris larvae 

 fixing together on the same spot, but rather by the Cypris larvae 

 seeking out adolescent individuals of their own species and 

 fixing on or near them. Now, if we suppose that the Cirripedes 

 have passed through a condition of protandric hermaphroditism 

 similar to that of the Isopoda Epicarida, it is clear that a slight 

 modification of the sexual instinct of the larvae would lead to 

 the gregarious habit, while its retention in some individuals in 

 its original form accounts for their finding their way to the 



