ECHINODERMA. ECHINOIDEA 



123 









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On the surface of some irregular sea-urchins belonging 

 to the sub-order Atelostomata (p. 129) there are bands 

 which appear to be nearly smooth, but are covered with 

 very minute tubercles; in the living state they bear slender 

 modified spines. These bands are termed fascioles, and 

 their position varies in different a 



genera ; sometimes they form 

 a ring beneath the anus (e.g. 

 Micraster, fig. 50, c), when 

 they are said to be sub-anal; in 

 other cases they encircle the 

 rosette formed by the petaloid 

 ambulacra (e.g. Hemiaster) 

 and are said to be peripe- 

 talous ; or they extend round 

 the margin of the test (e.g. 

 Epiaster). 



On the lower surface there 

 is an opening in the corona 

 known as the peristome (fig. 

 50, a) ; this is closed by a membrane in the centre of which 

 is the mouth. The peristomal membrane is sometimes 

 (e.g. Cidaris) completely covered with rows of scaly plates, 

 but more usually bears small isolated plates only. The 

 peristomal membrane and its plates are usually lost in fossil 

 specimens. The shape of the peristome varies in different 

 genera, it may be circular, oval, pentagonal, or decagonal ; 

 at its margin there are frequently ten notches, by which 

 the five pairs of gills or branchiae pass to the exterior. 

 The peristome is usually larger in the regular than in the 

 irregular echinoids. In some irregular echinoids belonging 

 to the sub-order Atelostomata (p. 129) the parts of the 



Fig. 50. Under surface of Micra- 

 ster cor-anguinum from the 

 Upper Chalk, showing fasciole. 

 a, peristome ; &, periproct ; c, 

 fasciole. x f . 



