Chap, ix.] APICAL AREA OF ECHINUS. 291 



This is best studied in a regular form sucli as the 

 ordinary sea-urchin (Echinus), where it is found to 

 consist of two sets of five plates (Fig. 121 ; A), one of 

 which is radial and one interradial in position. The 

 former are spoken of as the ocular plates, and are 

 perforated by an ordinarily single orifice, through 

 which a tentacle protrudes. The interradial plates, 

 which are similarly perforated, and which are generally 

 larger than the radial, are called the genital plates, 

 from the fact that they have become secondarily 

 modified to serve as the means of exit of the generative 

 products ; for the comprehension of the morphology of 

 this apical part of the test it is, however, best to 

 strictly limit our nomenclature to the terms radial and 

 interradial. One of these interradial plates, that 

 which lies to the right of the anterior ambulacrum or 

 radial plate, as seen in Fig. 121, is specially modified 

 to serve as the entrance to the madreporic or stone- 

 canal. It is distinguished by the name of Hiadrepo- 

 rite, and is characterised by its larger size, and its 

 perforation by a number of minute orifices. 



Within the circlet of these radial and interradial 

 plates of the apical area, there is a space which is 

 ordinarily covered by a number of small irregularly- 

 shaped calcareous plates, in or near the middle of 

 which there is an orifice, the anal opening of the 

 digestive tract ; sometimes, as in Echinocidaris, the 

 number of anal plates is much smaller, and in the 

 genus just mentioned there are very often not more 

 than four ; in Diadema the anal area is very nearly 

 completely membranous, and the anal orifice is placed 

 at the end of a projecting tube. Among the irregular 

 echinids the anus leaves its apical position, and opens 

 either some way posteriorly on the upper surface, as 

 in Rhyncopygus, or at the margin of the test, as in 

 Echinolampas, or on the lower surface and quite close 

 to the mouth as in Echiiioneus. 



