THE CIRRIPEDIA 



the reverse is probably the case with regard to the class Crustacea 

 as a whole. It has been held by some authorities that the dioecious 

 state is the more primitive in the Cirripedia also, and the attempt 

 has been made to show that the more degraded types of the dwarf 

 males resemble in their structure the Cypris larva, which is supposed 

 to represent the ancestral form of the Cirripedes before the assump- 

 tion of the sedentary habit. As a matter of fact, however, the 

 points of resemblance between these males and the Cypris larva 

 are very slight. Such characters as the incomplete alimentary 

 canal and the reduced cirri show that these males are not primitive 

 but degenerate forms, and that a phylogenetically older stage is 

 represented by species like S. peronii, in which the males resemble 

 the hermaphrodite individuals. It is very probable that the differ- 

 entiation of the sexes began among species in which, on account 

 of their deep-sea habitat (as in most species of Scalpellutri) or 

 burrowing habits (as in the Acrothoracica), cross -fertilisation 

 between hermaphrodites was difficult. It is common, in many 

 species of Pedunculata, to find young individuals attached to the 

 peduncle of older ones, and it is but a step further to find these 

 younger individuals, attached in the most favourable position for 

 fertilisation to the mantle-margin of the others, performing only 

 the function of males. It is more difficult, perhaps, to imagine 

 why in so few cases is the separation of the sexes complete, but 

 possibly the retention of male organs by the large individuals may 

 be regarded as a precaution against a failure in the succession of 

 short-lived males. 



DEVELOPMENT. 



With a very few exceptions, the Cirripedia are hatched from 

 the egg in the nauplius stage, and all pass through a later larval 

 form, known, from the superficial likeness of its bivalved shell to 

 that of an Ostracod, as the Cypris stage. The nauplius (Fig. 73) has a 

 somewhat peculiar and characteristic structure, subject, for the most 

 part, to but slight modification in the different groups. The dorsal 

 shield is produced at the antero-lateral corners into a pair of tubular 

 horns, sometimes of great length. Each horn has, at its base, a 

 pair of unicellular glands (Fig. 73, dr) which discharge their secretion 

 through its open tip. The posterior end of the shield is rounded, at 

 least in the earlier stages, but immediately beneath it arises a long 

 spine directed backwards. A large process projecting downwards 

 and backwards from the ventral surface contains, in the later stages 

 of development, the rudiments of the trunk and its appendages. It 

 must therefore be regarded as the posterior end of the body, and 

 the fork which terminates it must be compared with the " caudal 

 furca," although the anal opening (A) is well in front of it on the 

 dorsal side. The usual three pairs of limbs present the character- 



