382 THE INVERTEBRATA 



very degenerate males, the large individuals are without testes, so that 

 the sexes are separate. The function of the complemental males is 

 probably the effecting of cross-fertilization, for the species which 

 possess them are of solitary habit. The phenomenon perhaps arose 

 from the settling of young hermaphrodite individuals on the stalk of 

 old ones, which is common in stalked barnacles. 



Balanus (Fig. 257 B), the common acorn barnacle, differs from 

 Lepas in the lack of a stalk, and in having an outer wall of skeletal 

 plates homologous with some of the extra pieces on the capitulum of 

 Scalpellum . 



Order ACROTHORACICA 



Cirripedia of separate sexes ; with an alimentary canal ; fewer than six 

 pairs of thoracic limbs; and no abdominal somites; permanently 

 sessile on the preoral region, in which the antennules are absent and 

 the cement glands much reduced. 



These are minute creatures whose females live in hollows which 

 they excavate in the shells of molluscs, while the males are degenerate 

 and have the same relation to the female as have those of the species 

 of Scalpellum in which the sexes are separate. 



Alcippe, British, lives in the columella of whelks, etc. 



Order APODA 



Hermaphrodite Cirripedia; without mantle, thoracic limbs or anus; 

 whose body is divided by constrictions into rings. 



Proteolepas, the only known member of the order, is a small, 

 maggot-like animal found by Darwin in the mantle cavity of the 

 stalked barnacle Alepas. The antennules, by which it is attached, and 

 the mouth parts, are those of a cirripede. Since the mouth is terminal, 

 at least some of the more anterior of the eleven rings cannot represent 

 somites. 



Order RHIZOCEPHALA 



Cirripedia which are parasitic, almOst exclusively on decapod Crus- 

 tacea ; have at no time an alimentary canal ; and in the adult neither 

 appendages nor segmentation; make attachment in the larva by an 

 antennule; and are in the adult fastened to the host by a stalk from 

 which roots proceed into the host's tissues. 



Sacculina (Figs. 258-261), parasitic on crabs, is the best known 

 example of this group. Its life history is a very remarkable one. It 

 starts life as a Nauplius (Fig. 258 A), with the characteristic frontal 

 horns of cirripede Nauplii but without mouth or alimentary canal. 

 The Cypris larva (Fig. 258 B) clings to a seta of a crab by one of its 



