396 ZOOLOGY SK<T. 



the peripheral part of the peristome, a pair opposite each inter- 

 ambulacral area. 



When the spines are removed, the body is found to be enclosed 

 in a rigid globular shell, or corona (Fig. 322) as it is termed, 

 formed of a system of plate-like ossicles, the edges of which fit 

 accurately and firmly together, and the surfaces of which are 

 ornamented with the rounded elevations or tubercles for the articu- 

 lation of the spines. These plates are arranged in ten zones, each 

 consisting of two rows, running in a meridional direction from the 



-6 



7 u- 



G 



FIG. 322. Corona of Echinus esculentus, from the aboral surface, showing the arrangement 

 of the plates of the corona. 1, the anus ; 2, pcriproct, with irregular plates ; 3, the raadre- 

 porite ; 4, one of the other genital plates ; 5, au ocular plate ; (5, an inter-ambulacral plate ; 

 7, an ambulacral plate ; 8, pores for the protrusion of the tube-feet ; 9, tubercles. (After 

 MacBride.) 



edge of the peristome to the neighbourhood of the periproct. Of 

 the zones of plates there are two sets, each consisting of five, the 

 members of which alternate with one another. In the case of one 

 of these sets of zones the ambulacral zones or ambulacral areas 

 already referred to each of the plates is perforated towards 

 its outer end by two minute pores, the ambulacral pores, for the pro- 

 trusion of the tube feet. In the other five zones, the inter-ambu- 

 lacral zones or areas, the plates are not perforated. At its 

 anal end each area, ambulacral or inter-ambulacral, ends in a 

 single apical plate, so that the periproct is surrounded by a ring of 



