338 



ARTHROPODA. 



" Barnacles." In these (fig. 198) the animal differs from the 

 Sessile Cirripedes in having its anterior extremity greatly 

 elongated, forming a stalk or " peduncle " by which it 

 is fixed to some foreign object. At its free extremity 

 the peduncle bears the "capitulum," 

 which corresponds to the shell of the 

 Balanoids, and is composed of various 

 calcareous pieces, united by a mem- 

 brane, moved upon one another by ap- 

 propriate muscles, and protecting in 

 their interior the body of the animal 

 with its various appendages. The 

 peduncle is cylindrical, of varying 

 length, flexible, and furnished with 

 proper muscles. In some species the 

 peduncle is naked, and cannot be pre- 

 served in the fossil condition ; but in 

 other cases the peduncle is furnished 

 with calcareous scales (Loricula and 

 Turrilepas, fig. 200), in which case it 

 is readily preserved. The " capitulum " 

 (fig. 199), as '.! before . said, corresponds 

 with the shell:, of : the Balani, and is 

 generally much flattened. It consists 

 ordinarily of five or more valves united 

 to one another by membrane, usually 

 with marked interspaces ; but the 



valves may be rudimentary or want- 

 Fig. 198. Anatifa lepas, a . , . , .. ., , , 



recent Peduncuiated cimpede. ing, and the entire capitulum may be 



s T cuU W d e e r tachT ^ *" membranous. The parts of the capitu- 



lum correspond ideally with the parts 



of the shell in the Balanoids. In the latter, however, the 

 shell is for the most part composed of the " compartments," 

 and the " operculum " is comparatively small and insignifi- 

 cant. In the Lepadoids, on the other hand, the valves which 

 correspond with the operculum of the Balanoids are dispro- 

 portionately developed, and the valves which correspond with 

 the compartments of the Balanoids are much less conspicuous, 

 and are often partially absent. The most important and per- 



