230 CRUSTACEA. 



the sexes in all other Crustacea, ^^-e regard this hermaphroditism as 

 secondarily acquired in consequence of the attached manner of life. 

 We must assume that the free-swimming ancestors of the Cirripedia 

 were of separate sexes, and that hermaphroditism was only gradually 

 acquired after the fixed sedentary mode of life had become fully 

 established as a characteristic of this group. While, in the Balanidae 

 and Khizocephala, hermaphroditism has become the exclusively 

 prevailing condition, there is a tendency in many groups of the 

 Lepadidae to retrogression in the direction of the separation of 

 the sexes. Male forms here appear either side by side with herma- 

 phrodite individuals, being then called complemental males, or else, 

 in cases of the complete separation of the sexes, in company with 

 true females. These complemental males are always smaller tlian 

 the hermaphrodite individuals or the females ; they are found 

 attached parasitically to the bodies of the hermaphrodites or females. 

 In isolated cases, the shape of the body in the male deviates only 

 slightly from that in the hermaphrodite (Scalpellum villosum and 

 S. Peronii), but in other cases there is striking sexual dimorphism 

 in this respect, the male undergoing a process of degeneration in 

 which the calcareous portions of the skeleton, the limbs, the mouth, 

 and the alimentary canal have been lost, thus sinking to a very low 

 grade of organisation, and becoming actually a dwarf or complemental 

 male. The following degrees of degeneration of the male are found 

 in the genus Scaliiellum (Hoek, No. 46). 



I. True hermaphrodite forms [Scal2)ellum balanoides, Hoek). 



II. Large hermaphrodite forms with small complemental males, 

 (a) The males resemble the hermaphrodites in structure. 



The division into capitulum and peduncle is recognisable, mouth 

 and alimentary canal are present [Scalpellum villosum, S. Peronii). 



(h) The males have degenerated ; they are without mouth and 

 intestine ; without a shell, or have only a vestigial shell ; without 

 a peduncle [Scalpellum vidgare, S. rostratum). 



III. Separate sexes. The female is large and resembles the 

 hermaphrodite individuals of the other species. The male is very 

 small [Scalpellum ornatum, S. regium, Hoek, etc.). 



The Abdominalia {Alcippe, Cryptophudus) show a differentiation akin to that 

 of this last groiqi. Here also we find separation of the sexes with highly developed 

 sexual dimorphism. The complemental males appear very much reduced. They 

 have no tendril-like feet, no mouth, and no alimentary canal. In other respects, 

 if we take into account the reduction that has commenced, their structure can 

 be traced back to that of the female. 



A condition resembling that above described for Scalpellum is found in the 



