ECHINOIDEA 257 



oblong, " irregular " forms the posterior genital plate is lacking. 

 The right anterior genital plate also has the function of a 

 madreporite ; in the Irregularia it is usually prolonged backwards, 

 separating the other plates of the apical system (Fig. 141, 2). 

 A small pore is also found in the ocular plates, through which the 

 end of the radial water- vessel protrudes as a small, unpaired 

 feeler, which does not, however, carry an eye, such as is the case 

 in the sea-stars. Inside the circle of the 10 apical plates there is 

 situated, in the round or " regular " forms, a small area covered 

 with irregularly arranged, movable small plates, the anal area or 

 'periproct, and herein the anal opening is situated, often some- 

 what eccentrically towards the right posterior edge. One of the 

 plates of the anal area may be conspicuously larger than the 

 rest of them, and is then designated the anal plate (Fig. 141, 1). 

 In the irregular Echinoids the anal area has moved outside the 

 apical area to the posterior edge of the test or even down to the 

 under side, close to the mouth. The mouth is on the under side 

 of the test ; in the regular forms it is situated in the centre, in 

 a small, round, soft-skinned area, the mouth area or peristome, 

 which is usually covered, more or less, by small plates, but may 

 sometimes be quite naked, excepting for a pair of plates off each 

 ambulacral area, the buccal p)lates, each of which carries a tube- 

 foot. In some of the Irregularia the peristome has moved 

 tow ards the anterior end of the test ; it is in this group of 

 Echinoids usually transverse -oval, wholly covered by small 

 plates, none of which carry tube -feet. 



To the test and peristome various appendages or organs 

 are attached, namely spines, pedicellarise, sphaeridise, tube- 

 feet, and gills. The spines are articulated to tubercles on the 

 test ; these latter generally have the shape of a truncated cone, 

 the " boss," with a rounded head or condyle, the '' mamelon," 

 on the top. At the base of the spine is found a serrated, ring- 

 shaped widening, the " milled ring ", from which proceed muscles 

 which attach to the test round the tubercle, and by means of 

 which the spine can be moved in all directions. In some forms 

 an elastic string proceeds from the centre of the base of the spine, 

 attached with its other end in a groove in the top of the mamelon. 

 Such a tubercle with a central hole in the mamelon is designated 

 ''perforate''. On the larger tubercles, the primary tubercles, 

 there is a distinct platform under the mamelon ; it may be 

 indented in the edge, in which case the tubercle is said to be 

 '' crenulate'\ The larger tubercles are surrounded by a more or 

 less distinct, in some forms (especially the Cidarids) distinctly 



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