. Elementary text-book of zoology. Zoology. EXOSKELETON. 271. cases the dermal muscular system is strongly developed, and has the 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 processes and spicul
. Elementary text-book of zoology. Zoology. EXOSKELETON. 271. cases the dermal muscular system is strongly developed, and has the 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 processes and spicules (fig. 211). The exoskeleton in the Sea-urchins is iminoveable. 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 (ambulacral plates, fig. 212) ; the other be- longs to the inter-radii, and the plates are unpierced (the interaiubulacral plates, fig. 206, R, J). Near the apical pole, which in the Grinoidea 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 interarnbulacral 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 Criiwidea, 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 appendages of the dermal armour, the numerous and variously s
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
