106 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 hermaphroditism 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 Rhizocephala, 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, ete., these individuals being also potential 
hermaphrodites, which are arrested in development, though not 
so completely as in the Rhizocephala, 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 
sinilar 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 
