ix PHYLUM ECHINODERMATA 393 



When the first ossicles are definitely formed they present the 

 following arrangement (Fig. 319). In the middle of the abactinal 

 surface is a single central plate (dors). Around this are five basals 

 (bas) one of which becomes merged into the madreporite. External 

 to these, five radials (rad) appear somewhat later. At the end of 

 each developing arm is a single terminal or ocular plate (term), 

 which is carried outwards as the ambulacral and adambulacral 

 ossicles of the arm are developed, supporting the corresponding 

 eye and tentacle. A ring of secondary radials or infra-basal* (sec. 

 rad) is developed between the radials and the central. In the 



sec.ra.tt 



FIG. 310. Diagram showing the relations of the chief plates of the apical system in the young 

 Starfish, an. anus ; bos. basals ; dors, central ; -uiadr. madreporite ; rod. radials ; sec. rad. 

 secondaiy radials (infra-basals). 



adult, by the intercalary development of numerous additional 

 ossicles, these primary plates of the apical system, as it is termed, 

 lose their original arrangement, and become no longer recognisable. 



2. EXAMPLE OF THE ECHIXOIDEA. 

 A Sea-Urchin. (Strongyloccntrotus or Echinus.) 



General External Features. The Sea-Urchin (Figs. 321 and 

 322) is globular in shape, but somewhat compressed in one direc- 

 tion, so that two poles are distinctly recognisable. At one of these 

 the degree of flattening is greater than at the other ; this is the 

 oral pole, the opposite pole being termed the anal or aboral. At 

 the oral pole is a rounded aperture, the mouth, through which may 

 be seen projecting five hard white points, the extremities of the 



