86 CRUSTACEA—CIRRIPEDIA CHAP. 
remarkable variation in the sexual constitution of some of the 
species. The great majority of the Pedunculata and all the 
Operculata are hermaphrodites, which habitually cross-fertilise 
one another, and this they are well fitted to do, since they all 
live gregariously and are provided with a long exsertile penis 
for transferring the spermatozoa from one to the other. In 
Pollicipes, however, the individuals of which often live solitarily, 
it appears that self- fertilisation may occur. In Scalpellwm 
Fic. 55.—A, Complemental male of Scalpellim peronii, x 20; B, hermaphrodite 
individual of S. vulgare, x 2. a, Complemental males, in situ; 6, rostrum. (A, 
after Gruvel ; B, after Darwin.) 
three different kinds of sexual constitution may oceur: (1) 
According to Hoek in S. balanoides, taken by the Challenger, the 
individuals are ordinary cross-fertilising hermaphrodites. (2) In 
the great majority of species, including the common S. vulgare, 
as originally described by Darwin, and since confirmed by Hoek 
and Gruvel,' the individuals are hermaphrodite, but there are 
present affixed to the adult hermaphrodites, just inside the 
opening of the valves in a pocket of the mantle, a varying 
number of exceedingly minute males, called by Darwin “ com- 
plemental males.” These tiny organisms are really little more 
1 Arch. Biol. xvi., 1899, p 27. 
