6 COMPABATIVE ANATOMY. 



shell, composed of numerous upwards of six hundred calcined 

 pieces, arranged in a definite pattern. The shape of the test is 

 that of a sphere with flattened poles. One pole however, the 

 oral one, is much more flattened than the other, the anal one. 

 With the exception of small circum-anal and circum-oral areas 

 the remainder of the body, ' corona ' as it is called, is composed 

 of numerous calcified plates, disposed in twenty longitudinal 

 series. These are furthermore arranged in ten double rows, 

 radiating from one pole to the other. The five double rows of 

 plates, which are perforated by the external apertures of small 

 canals, constitute the ambulacra, while the remaining five 

 double rows, alternating with the foregoing, and imperforate, 

 form the inter-ambulacra. The ambulacral and iriter-ambu- 

 lacral plates in converging towards the poles of the shell be- 

 come much smaller, and do not quite reach the poles, but 

 leave small areas round the mouth and anus, in which the 

 calcified plates have a different arrangement. 



In the neighbourhood of the anus this space is occupied by 

 two concentric rows of plates, the anal plates. The much larger 

 circum-oral space the 'peristoma is in the fresh specimen filled 

 up by a leathery membrane strengthened by irregularly shaped 

 calcified plates, in the centre of which is the mouth. Each 

 ambulacral row of plates is separated from the adjacent row of 

 inter-ambulacral plates by a tolerably straight but not very well 

 defined suture. On the other hand the suture separating the 

 two rows which together make up an ambulacrum or an inter- 

 ambulacrum is a zigzag one: being formed by the alternation of 

 the triangular internal extremities of the component plates. 

 There is consequently such a suture running down the middle 

 of each ambulacrum and inter-ambulacrum. The plates com- 

 posing an ambulacrum are pentagonal, and transversely elongate. 

 Each is perforated by six obliquely disposed apertures or pores, 

 affording a passage to the tube-feet : and each carries on its 

 surface conical tubercles varying in size. Furthermore each 

 plate is subdivided by faintly visible oblique transverse sutures 

 into three primary divisions or pore-plates, each pore-plate con- 

 taining a pair of obliquely set pores : and the whole six aper- 

 tures are situated close to the outer or inter-ambulacral edge of 

 the plate. ' There is therefore a pore-plate or subdivision of 



