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CRUSTACEA. CIRRIPEDIA 



SUB-CLASS V. CIRRIPEDIA 



The Cirripedes include the barnacles, acorn-shells, etc. 

 — forms which differ considerably in 

 appearance from the other crusta- 

 ceans and were for a long time re- 

 garded as molluscs. The body is 

 completely enclosed in a fold of the 

 skin, which commonly secretes a 

 calcareous shell. The animal, in the 

 adult state, is fixed to a foreign 

 object by the anterior end of the 

 head, either directly or by means of 

 a stalk or peduncle. The segmenta- 

 tion of the body is indistinct. The 

 head is not well marked off from 

 the thorax ; it bears one or two 

 pairs of antennae, one pair of man- 

 dibles, and two pairs of maxillae. 

 The thorax has usually six pairs of 

 biramous feathery limbs. The abdomen is rudimentary 

 and without appendages. Heart and vascular 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. 

 137, 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 ; its shell consists of a tube formed 

 of six pieces, within which the scuta and terga are placed. 



Fig. 137. Lepas australis, 

 Recent, a, scutum ; 

 b, tergum; c, carina ; 

 d, peduncle. Natural 

 size. (After Darwin . ) 



