336 COMPARATIVE ANATOMY chap. 



sucli a way as to hide it. These plates are by some regarded as 

 orals. 



In the Cdincrata (Fig. 299) five supposed oral plates {or) can almost 

 always be distinguished in the centre of the richly and rigidly jjlated, 

 often highly arched, tegmen. They close together firmly over the 

 mouth. The posterior oral is larger than the rest, and presses in 

 between them. 



As far as is known, in the Articulata (IcJiihyocrinoidea) also, five 

 orals can be distinguished at the centre of the richly but loosely 

 plated tegmen. But, in this case, they are separate, and surround an 

 open mouth. The posterior plate is larger than the rest. 



In the Gamdiculata (with the exception of the above-named genus 

 Rhizocrlims) the orals are altogether wanting in the adult. 



In the Blastoidea the oral region is covered by a roof consisting 

 of numerous small plates usually without definite arrangement, which 

 are continued as covering plates over the ambulacra. In a few forms, 

 however, and especially in StepJtanocrinus, five orals can be made out. 

 In Stephanocriims these five interradial orals, resting on the inter- 

 radials {i.e. the deltoid pieces), form a closed pyramid over the oral 

 region. 



In many Cystidea, also, the mouth is arched over by an oral 

 pyramid. In LUjatliocijdh, the five oral plates forming this pyramid 

 are more or less equal in size, but in species of the genera Spha'roim, 

 Ghjpiospha'ra, and Pirocystis the posterior oral is, as in so many 

 Camerata, larger than the rest. In the six-rayed Cystid Canjocrinus 

 this latter is the case, one of the six orals having shifted from behind 

 forward between the other five, which surround it symmetrically. 



In the Ophiuroidea, on the oral (lower) side of the disc, there is 

 in each interradius a plate, usually distinguished by greater size. One 

 of the.se plates, which ai'e called buccal shields (Fig. 245, p. 300), 

 is, as madreporite, perforated by the pores of the watei"-vascular 

 system. In the pentagonal larva of Amphiura these buccal shields 

 appear at the edge of the oral side. They have been homologised, 

 probably correctly, with the orals of the I'clmatozoa. 



In the Asteroidea, on the lower surface of the disc at the edge of 

 the mouth, in each interbrachial region, there occurs a skeletal plate 

 of very various shape, which is called the odontophore (Fig. 310, p. 

 352). These plates, which might be described as the proximal or 

 basal plates of the interbrachial system, may correspond with the 

 orals of the Pelmatozoa and the oral shields of the Ophiuroidea, although 

 they may be pushed below the surface by the oral plates (the first 

 pairs of adambulacral plates), and are usually completely covered 

 externally. The}' arise early in the larva of Asterias (after the five 

 terminal plates, the five basals, the apical central plate, the ten oral 

 ambulacral plates, and twenty other ambulacral plates are formed), 

 interbrachially between the oral ambulacrals. 



Orals have not been discovered in the Echinoidea. AVhether 



