THE CIRRI P EDI A 139 



support this view. The resemblance of the bivalved shell of the 

 Cypris larva to that of an Ostracod extends to such characters 

 as the asymmetry of the valves and the presence of fronto-lateral 

 glands near the margins of the shell ; but the rest of the organisa- 

 tion differs widely from that of the Ostracoda. Among the 

 characters which are probably significant of the isolated position 

 of the Cirripedia are the difference in position of the genital 

 apertures in the two sexes and the fact that the female openings are 

 placed farther forward than in any other Crustacea. The nature of 

 the larval development, especially the sudden transition between 

 the sharply contrasted nauplius and Cypris stages, indicates a 

 high degree of specialisation, and the structure of the adult animals 

 is in many respects so clearly correlated with the sessile habit and 

 the mode of obtaining food as to afford little help in deciphering 

 their phylogeny. It may be noted here that, like other groups 

 of sedentary organisms, the Cirripedia show a tendency to the 

 assumption of a superficial radial symmetry, which becomes very 

 marked in the Operculata. 



The Cirripedia were divided by Darwin into three orders, 

 Thoracica, Abdominalia, and Apoda. The order Abdominalia 

 contained the single genus Cryptophialus, which Darwin, misled by 

 a superficial segmentation of the body, supposed to possess ap- 

 pendages on the abdominal region. It has been shown, however, 

 that the appendages in question are really thoracic, and that 

 Orypiophialus does not differ in this respect from Alcippe, which 

 Darwin (although recognising the resemblance between the two 

 genera) placed among the Thoracica. In the classification of 

 Gruvel, adopted here, these two genera and some allied forms are 

 grouped together as Acrothoracica, but it is to be noted that the 

 differences separating this order from the Thoracica are probably 

 less important than those distinguishing the other orders, and 

 that the recently- described Koleolepas of Stebbing helps to unite 

 the two. 



The genus Proteolepas, the sole representative of the order 

 Apoda, is still known only from Darwin's description of a single 

 specimen and its affinities are obscure. In the absence of a mantle 

 it differs widely from all other Cirripedia. 



The Rhizocephala are plainly characterised as Cirripedia by 

 their larval stages, but much remains to be done in elucidating the 

 relationships of some of the genera at present referred provisionally 

 to this order. The Ascothoracica form another order, established 

 since the date of Darwin's work, but many points in their morpho- 

 logy and development are still too imperfectly known to allow of 

 their systematic relations being precisely defined. 



