Invertebrate Gallery of the Indian Museum. I05 



The poriferous radii and the imperforate interradii do 

 not meet at the apex of the test, but the apex of the test 

 is occupied by a star-shaped area formed by ten plates 

 surrounding a central orifice. This central (apical) orifice 

 is the " periproctal area ", and in life it is covered in with 

 small scaly plates, and is perforated by the anus. Of the 

 ten plates which surround the periproctal area, five are seen 

 to occupy the summits of the radii, while five much larger 

 ones occupy the summits of the interradii : those which 

 surmount the radii are known as " radial plates" (formerly 

 termed ** ocular plates"), while those which surmount the 

 interradii are known as "basal plates" (formerly termed 

 "genital plates "). Each basal plate is conspicuously per- 

 forated by a large pore fo r the duct of the underlying geni- 

 tal gland, and one of the basal plates is much enlarged and 

 spongy. This large spongy basal plate is the madreporic 

 plate, through which the sea-water filters into the stone- 

 canal, and so to the water-vascular system. 



Looking from below we see that neither the radii nor 

 interradii quite meet at the lower pole of the shell, but 

 we find there a large orifice. This is known as the peris- 

 tomial area, and in life it is filled up with a scale armour 

 df little plates, except in the centre where the mouth lies. 



The intact peristomial area, with the mouth and the five 

 large teeth characteristic of the Regular Echinoids, are 

 seen in the neighbouring specimen of Dorocidaris (Case 

 28). 



The arrangement of the mouth and of the five jaws, with 

 their " pyramids ", which work on the " braces " and *' com- 

 passes" and enclose the sharp-pointed teeth, is shown in 

 the neighbouring dry preparation of the mouth-parts of 

 Salmacis (Case 28). 



Alongside of the jaws of Sahnacis is shown half of the 

 test of the individual from which the jaws were removed : 

 this preparation is meant to exhibit the internal upstand- 

 ing processes (or *' auricles") of the ambulacral plates, which 

 support the " pyramids " and give attachment to the 



