CRUSTACEA. CIRRIPEDIA. 



189 



b 



means of a stalk or peduncle. The head is not well- 

 marked off from the thorax ; it bears two pairs of antennae, 

 one pair of mandibles, and two pairs of maxillae. The 

 thorax has usually six pairs of bira- 

 mous feathery limbs. The abdomen 

 is rudimentary. A heart and vas- 

 cular system are absent ; nearly all 

 forms are hermaphrodite. The shell 

 consists of several pieces ; in Lepas 

 (which possesses a stalk) there are 

 five, two are placed on each side 

 of the body, those near the stalk 

 being termed the scuta (fig. 5 2 a), 

 those at the upper end the terga 

 (b), and there is also one unpaired 

 part placed dorsally, the carina (c). 

 Balanus has no stalk ; in this the 

 shell consists of a tube formed of six 

 pieces, in the upper part of which 

 the scuta and terga are placed. 



FIG. 52. Lepas austra- 

 lis, Recent, a, scutum ; 

 b, tergum; c, carina; d, 

 peduncle. Natural size. 

 (After Darwin.) 



Distribution of the Cirripedia. 



The Cirripeds are all marine, and the greater number 

 are found in shallow water. Balanus is especially charac- 

 teristic of littoral regions. At a depth of greater than 

 1000 fathoms, only two genera, Scalpellum and Verrucosa 

 have been found, but these are not confined to deep water. 

 The earliest genus is Turrilepas, which ranges from the 

 Ordovician to the Devonian. Pollicipes commences in 

 the Rhaetic, Scalpellum in the Cretaceous, and Balanus in 

 the Eocene ; all three are still living. 



