496 



CRUSTACEA. 



which a many-valved shell is typically developed. The abdomen is 

 rudimentary ', and the thorax usually carries six pairs of multiartic- 

 ulate limbs ("cirri"), each of zvhich consists of a protopodite carrying 

 a long, jointed e?idopodite and exopodite. 



The typical Cirripedia are distinguished by the fact that in the 

 adult condition they are permanently fixed to some solid object by 

 the anterior extremity of the greatly metamorphosed head ; the first 

 three cephalic segments being much developed, and enclosing the 

 rest of the body. The larva is free and locomotive, and the subse- 

 quent attachment, and conversion into the fixed adult, is effected 

 by means of a peculiar secretion, or cement, which is discharged 

 through the antennae of the larva, and is produced by special cement- 

 glands. In the Cirripedia, therefore, the head of the adult is per- 



Fig. 354. — Morphology of Cirripedia. a, Lepas ficctinata, one of the Barnacles, one side of 

 the shell being removed, enlarged four times : c, Peduncle ; d, Cement-duct ; o, Ovary ; s, 

 Ovisac; 7', Vas deferens; /, Penis. B, Pcccilasma fissa, enlarged five times; c, Peduncle, 

 c, Balanus balanoides, viewed from above, of the natural size, d, Balanus ti7iti7inaln<lu)ii, 

 with the shell on one side removed to show the animal ; «, One of the valves ("scutum") of the 

 operculum ; b, Another valve (" tergum ") of the operculum. (After Darwin and Pagenstecher.) 



manently fixed to some solid object, and the visceral cavity is pro- 

 tected by an articulated calcareous shell, or by a coriaceous envelope 

 (fig. 354). The posterior extremity of the animal is free, and can 

 be protruded at will through the orifice of the shell. This extremity 

 consists of the rudimentary abdomen, and of six pairs of forked, 

 cirrated limbs, fringed with hairs, which are attached to the thorax, 

 and serve to provide the animal with food. The two more important 

 types of the Cirripedia are the Acorn-shells (BalanidcB and Ver?'u- 

 cidce) and the Barnacles (Lepadidce). In the former the animal is 

 sessile, the larval antennae, through which the cement exudes, being 

 embedded in the centre of the membranous or calcareous " basis " 

 of the shell. In the latter the animal is stalked, and consists of a 

 " peduncle " and a " capitulum." The peduncle consists of the 



