. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. CIRRIIOPODA. 685. the five valves just described, there are other eight smaller calcareous plates arranged around the junction of the peduncle with the shells. The shells of the Balanids present several striking peculiarities of structure, and, in their mode of growth, offer to the physiologist an interesting subject for investigation. They form truncated cones, the bases of which, without the intervention of peduncles, are fixed to rocks, floating wood, integuments of marine animals, &c. These cones are composed of
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. CIRRIIOPODA. 685. the five valves just described, there are other eight smaller calcareous plates arranged around the junction of the peduncle with the shells. The shells of the Balanids present several striking peculiarities of structure, and, in their mode of growth, offer to the physiologist an interesting subject for investigation. They form truncated cones, the bases of which, without the intervention of peduncles, are fixed to rocks, floating wood, integuments of marine animals, &c. These cones are composed of several pieces, closely cemented together so as to admit of no motion between them, excepting during the process of enlargement of the shell. In the common acorn-shells (jig. 333), which cover our litto- Fig. 333. ral rocks and the bottoms of ships, there are seven of these pieces, six form- ing the walls, and one dis- coid, forming the base. The outer surface of the parietal valves is mark- ed by the lines of growth in such a manner as to give it the appearance of being com- posed of twelve pieces. These may be termed compartments. They are all conical. Six of them have their bases applied to the common base of the shell, and the other six are inserted between these, with their apices towards the common base. The first six we shall refer to under the name of the first series of compart- ments («, a, jig. 333) ; the other six constitute the second series (b, b, jig. 333). The opening in the summit of the cone is closed by an opercule composed of four shelly pieces so arranged as to leave a longitudinal fissure be- tween them, through which the arms are pro- truded (c, jig. 333). The two series of com- partments differ much from one another in their external aspect, owing to the differences in the directions and appearances of the lines of growth. The second series have a smoother surface, and are marked with very delicate lines, both longitudinal and transverse; they are also
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