ECHINODERMA TA—BLASTOIDEA . 



475 



The calyx is usually composed of 13 principal plates, firmly 

 united to one another, and arranged in three successive cycles, 

 represented by the three hasals, five radials and five deltoids or 



Fig. 1786. Eleacrinus verneuili, summit view, showing position of anus, double 

 spiracles and central vault-plates, covering peristome, X 2^, and portion of an ambu- 

 lacrum with outer plates, X ^- (After Etheridge and Carpenter.) 



mterradials. Resting upon the basals are five V-shaped, usually 

 equal radials (''forked plates") whose upper margins are more 

 or less deeply incised by the radial sinuses (Fig. 1788, a, 1791). 

 The term sinus is applied to the open space between the two prongs 

 or limbs of the plate. Succeeding arid alternating with the radials 

 and resting upon their prongs, are five interradial or deltoid plates 

 which, among different species, vary exceedingly in size (Fig. 

 1792, a, b, d). Only a part of the deltoids is exposed to view, their 

 sides being provided with flanges which are covered by the ends of 

 the ambulacra. The ambulacra fill the radial sinuses between the 

 prongs of the radials and vary in form from petaloid (wide in the 

 middle and tapering to each end), to narrow lanceolate (Fig. 

 1788, &). 



The open space or mouth opening in which the ambulacra meet 

 IS five-angled and central in position. This space is usually open, 

 but in well preserved specimens it is covered by a varying number 

 of minute summit plates (tegmen) either regularly or irregularly 

 arranged, but always leaving at the end of each ambulacrum a 

 small passage-way by means of which the food entered the mouth 

 opening beneath (Fig. 1786, a). Around the mouth are usually 

 five pairs of circular or slit-like openings or five single openings, 

 the spiracles. An additional opening {anus) is at times present 



