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 shaped spines and the pedicellariae must be mentioned. 



FIG. 212. Third ambulacrum 

 of a young; Toxopneustes <lro< - 

 lacheiisii of 3 mm (after Lov^n). 

 Ojj, Ocular plate ; P, primary 

 plates and tentacle pores. The 

 sutures of the primary plates 

 are visible on the plates ; Sw, 

 the tubercles to which the 

 spines are articulated. 



FIG. 213.- 

 Pedicella- 

 ria of a 

 Li- iuddans 

 (after Per- 

 rier). 



