CIRIUPEDIA. 279 



the larva ; g is the caudal end of the body ; / the ventral surface, 

 with the aperture of the shell from which the cirrigerous feet pro- 

 trude : e is the dorsal surface. The relative position of the retracted 

 soft parts to the multivalve shell is shown in Jig. 121. 



All the sessile Cirripeds are strongly defended by a multivalve 

 conical shell, closed by a four-valved operculum i^fig. 113). The base 

 of the shell is usually formed by a calcareous plate, and the walls are 

 apparently divided into twelve conical compartments, six of which 

 {ad), called "arete prominentes" and "parietes," rise from the margin 

 of the base, and terminate in a point at the free margin of the shell ; 

 whilst the other six {hh\ in the form of inverted cones, called "areae 

 depresste" and " radii," occupy the interspaces of the preceding series. 

 This calcareous citadel is divided into from four to eight pieces by 

 fine sutures. The piece at the part of the shell nearest which the 

 cirri (c) are protruded is called the " carina ; " the opposite piece is 

 the " rostrum." The sjTumetry or bilaterality of the shell is deter- 

 mined by these pieces, which have two lateral appendages, called 

 " alse." Three pieces are, in Octomeris, interposed, on each side, be- 

 tween the carina and rostrum. They are called " carino-lateral," 

 "lateral," and "rostro-lateral." The two first have a radius on the 

 carinal side, and an ala on the rostral side ; the third piece has a 

 radius on each side. Our common Acorn-shells {Balcmus, fig. 113) 

 have two pieces on each side, the " carino-lateral " and " I'osti'o- 

 lateral." Each piece consists of an outer and inner plate, united 

 by longitudinal plates. The whole shell has a cellular and or- 

 ganised texture, and its gradual expansion is provided for by the 

 successive growth and calcification of processes of the mantle which 

 penetrate the uniting sutures. The cone is lengthened and widened 

 below by successive additions to its base, and is widened superiorly 

 by the gradual increase in breadth of the wedge-shaped pieces of the 

 second or inverted series. In the Tuhicinella *, a parasitic balanid 

 of the whale, the compound shell is a long subcylindrical tube, re- 

 minding us of that of an amphitrite ; but the animal, in both sessile 

 and pedunculate Cirripeds, is fixed to the bottom of the shell with 

 the head downwards, and the head is, therefore, superior in the ordi- 

 nary pendent position of the barnacles {Lepas,fig. 121). 



The external integument consists of chitinef , which also forms the 

 animal basis of the shell J ; the calcareous part of the latter consists 

 of 99'3 of carbonate of lime, with 0*7 of phosphate of lime§. The 

 chitine is lined by a soft vascular corium, the formative organ of both 



• Prep. No. 279. t IX. 



X In Lepas and Pvllicipes, CCXXIII. p. 30. § IX 



T 4 



