156 LECTURE XIII. 



Most of the Cirripedes have their visceral cavity protected by a 

 calcareous shell composed of many pieces ; but in some, as the Otion, 

 the membranous or pallial investment of the viscera is protected only 

 by an elastic horny sheath, continued from the epidermal covering of 

 the peduncle. Two small calcai'eous bodies, developed in the sub- 

 stance of the outer envelope, just above the brachial fissure, are the 

 sole rudiments of a shell in this genus, the horny covering of which 

 is produced at its free extremity into two cylindrical processes. In the 

 genus Cineras, the external tunic is strengthened by five calcareous 

 bars, two at the ventral fissure, giving outlet to the arms, two along 

 tlie terminal margin of the tunic, and one along the dorsal aspect. 

 In the common Barnacle {Lepas anatifera) the calcareous matter 

 extends from five centres, so as to protect the whole of the body, 

 which is appended to the peduncle : the cephalic pair of valves, or 

 that which is attached to the peduncle and defends the head, is 

 the largest : the single dorsal piece has been compared by Cuvier, 

 who retained the Cirripedes among the MoUusca, with the sym- 

 metrical dorsal valve in the shell of the Pholas. All the valves are 

 strongly marked with lines of growth, formed by successive additions 

 to their margins, as in the shells of MoUusca. In the Pollicipes there 

 are other smaller calcareous plates arranged round the junction of tlie 

 body with the peduncle. 



All the sessile Cirripedes are strongly defended by a multivalve 

 conical shell. The base of the shell is usually formed by a calcareous 

 plate, and the walls are apparently divided into twelve conical com- 

 partments, six of which rise from the margin of the base, and ter- 

 minate in a point at the free margin of the shell ; whilst the other six, 

 in the form of inverted cones, occupy the interspaces of the preceding 

 series. This calcareous citadel is divided into six pieces by six 

 sutures : the symmetry or bilaterality of the shell is determined by 

 the doi'sal piece being actually what each of the six pieces of the first 

 series seem to be, viz. a simple triangular plate with its apex upwards : 

 the two lateral pieces on each side consist each of the erect and 

 inverted triangular piece closely united together : the ventral piece 

 consists of one erect and two inverted triangular pieces, united 

 inseparably in the mature Balanus. The whole shell has a cellular 

 and organised 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 su- 

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

 of the second, or inverted series. In the Tubicinella*, a parasitic 

 * Prep. No. 27 9. 



