136 &hinoderma. 



plates. One of them is largei' than tho otliovs, and is the 

 madreporite, for it exhibits, besides the sexual aperturo, a number 

 of other delicate pores, allowing of the entraneo of watcn- into the 

 stone canal which is attached at this point. Eacli of tlio iivo smallur 

 plates is similarly perforated by an opening, Bmallor thiui tho genital 

 pore, through which a nerve passes, to be distributed to tlu' skin in 

 the region of the aperture; this spot is particularly sensitive; the 

 plates are termed ocular plates, because it was formerly 

 believed that each bore an eye. The apical plates surround n. small 

 membranous region, the anal area, in wliicli the anus opens, 

 usually somewhat excentrically ; it is provided with small, movably- 

 connected, calcareous plates. The lower ends of the ambulacral 

 and interambulacral series of ossicles surround n. hivf^'i> space, the 

 peristome, on which also there is no conneeti'd skeleton, though 

 it is furnished with a certain number of la-rgt^r or smalk^r cal- 

 careous plates; the mouth is central. 



The skeletal plates in the regular Sea-ui'chins are a.rra.nged 

 on this plan; other less regular typos may be derivi'd from it. 

 A simple departure occurs in certain Sea-iircliins, whieli a,re 

 yet considered as "regular"; for although the shell is ovate, 

 instead of round, the general sti-ucture conforms to this type 

 (genus Echinometra) . The so-called Irregular Kc^a.-u re li i ns 

 are more aberrant; the whole of the anal area, has nioveil 

 from the circle of apical plates, into one of the inter-ra.dii, and lies 

 between two rows of interambulacral plates at some distanee from 

 the apex, occasionally even close to the oral area; tlu^ apical plates 

 draw together dorsally, and the regular structure of tlu^ shell may be 

 almost completely retaincul, even the spherical Form. The iiil.c^i'- 

 radius, in which the anal area li(^s, is termed poHt(M'ior. 

 The loss of radial symmetry is greater if, a,s in nia.ny IrvegiiJav 

 Sea-urchins (Fig. 90 jB), the mouth no longer lies in tlie centi'e of 

 the ventral surface, but more anteriorly; this lias a mai'keil (^Hec(. 

 upon the whole structure of the animal, since the nioutli lias mjt 

 moved into a radius, but I'ctains its position at the lower pole of tho 

 principal axis, where it is still the meeting-place of all I lie radii 

 and inter-radii. Hence, some important alterations must iiecoHHai'ily 

 result; the development of both radii and i)ii-(M'-ra,(lii is esseutiall}' 

 changed (see Fig. 90 B). Twenty rows of plaices may, liowtn'er, 

 still be noticed, a,s in the regular forms; a-nd the ociila.r a.nd 

 genital plates remain as befori!, excepting that the la,tt(a' a,ro usually 

 only four, or even fewer, in number. Many of the IiTc^^nilar lOcliirKjids 

 {see Fig. 90), are also peculiar in that the ambulacral plat(!S are 

 differently developed on the dorsal and vcintral sidos of the sliell, in 

 correspondence with differences in the tube-fciet, to be (h^scribed; 

 often, too, the ambulacral plates of the anterior radius differ JVom 

 the rest. 



