. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. UXOSKSLETON. 271 0. cases the dermal muscular system is strongly developed, and has tiie form of five pairs of bundles of longitudinal muscles, external to which is a continuous layer of circular muscular fibres covering the internal surface of the integument. In the Star-fishes and Brittle-stars a moveable dermal skeleton is formed on the arms consisting of calcare- ous masses (ambulacral ossicles), connected together like vertebrae, while the integu- ment of the dorsal surface is filled w
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. UXOSKSLETON. 271 0. cases the dermal muscular system is strongly developed, and has tiie form of five pairs of bundles of longitudinal muscles, external to which is a continuous layer of circular muscular fibres covering the internal surface of the integument. In the Star-fishes and Brittle-stars a moveable dermal skeleton is formed on the arms consisting of calcare- ous masses (ambulacral ossicles), connected together like vertebrae, while the integu- ment of the dorsal surface is filled with calcareous plates, and bears projecting pr/cesses and spicules (fig. 211). The exoskeleton in the Sea-urchins is immoveable. It consists of twenty meri- dional rows of solid calcareous plates immoveably connected together by their edges so as to form a firm shell, which is continuous except at the two poles, where it is interrupted by membranous structures. The rows of plates are ar- ranged in two groups, each with five pairs; of which the one group is radial in position and consists of plates pierced by the pores for the exit of the ambulacral feet (ambulacrdl plates, fig. 212); the other be- longs to the inter-radii, and the plates are unpierced (the interambulacral plates, fig. 206, R, J). Near the apical pole, which in the Crinoidea and the embryonic Echinoidea is occupied by a single plate (central plate), there is, in the Sea-urchins, a small area covered with minute calcareous plates and containing the anus. Around this area the five ambulacral and the five interambulacral rows terminate, each in a pentagonal plate; the former ending in the smaller radial ocular plates (fig. 206), the latter in the larger inter-radial genital plates. The Crinoidea, in addition to the dermal skeleton of the disc, possess a stalk, which is composed of pentagonal calcareous masses, arises from the dorsal side of the body, and becomes attached to firm sur- rounding objects. Amongst the a
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