.271 



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 (anibulacral 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 (ambulacral 

 plates, fig. 212) ; the other be- 

 longs to the inter-radii, and the 



plates are unpierced (the interambulacral plates, fig. 206, 

 JK, 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 

 P lates ( n g- 206 )> tne latter in the larger inter-radial 

 genital plates. The Crinoidea, in addition to the 

 dermal skeleton of the disc, possess a stalk, which i,s 

 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 Toxopnettstes droe- 

 bachemis of 3 mm (after Lcven). 

 Op, Ocu'ar 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.- 



riei)! P 



