500 



CRUSTACEA. 



is known, to have commenced their existence towards the close of 

 the Secondary period, the Chalk having yielded two species. Ver- 

 ruca Stromia is found in the Coralline and Red Crags (Pliocene), 

 in Glacial deposits, and in existing seas. 



The third family of the Cirripedia Thoracica is that of the Lepa- 

 didce or Pedunculated Cirripedes, commonly known as " Barnacles." 

 In these (fig. 357) the animal differs from the Sessile Cirripedes in 

 having its anterior extremity greatly elon- 

 gated, 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 membrane, 

 moved upon one another by appropriate 

 muscles, and protecting in their interior the 

 body of the animal with its various appen- 

 dages. The peduncle is cylindrical, of vary- 

 ing length, flexible, and furnished with proper 

 muscles. In some species the peduncle is 

 naked, and cannot be preserved in the fossil 

 condition ; but in other cases the peduncle 

 is furnished with calcareous scales (Loricula 

 and Turrikpas, fig. 359), in which case it 

 is readily preserved. The " capitulum " 

 (fig. 358), 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 inter- 

 spaces ; but the valves may be rudimentary 

 or wanting, and the entire capitulum may be 

 membranous. The parts of the capitulum 

 correspond ideally with the parts of the 

 shell in the Balanoids. In the latter, how- 

 ever, the shell is for the most part composed 

 of the " compartments," and the " oper- 

 culum " is comparatively small and insignifi- 

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

 correspond with the operculum of the Balanoids are disproportion- 

 ately developed, and the valves which correspond with the com- 

 partments of the Balanoids are much less conspicuous, and are 

 often partially absent. The most important and persistent of the 

 valves are the "scuta" (fig. 358, b), which protect the front part of 

 the body, and correspond with the valves bearing the same name in 



Fig. 357. — Lepas anserifera, 

 a recent Pedunculated Cirri- 

 pede. The lower figure shows 

 the scutum detached. 



