& ECH1N0DKEJIA. 



their aboral and adoral faces moulded so as to fit into or to receive 

 corresponding depressions or outgrowths on the aboral and adoral 

 surfaces of the vertebrae behind and in front. By these depressions 

 and projections the range of movement of the ossicles on their 

 neighbours is restrained within certain limits. On the upper 

 surface there is a single dorsal plate, on either side a spine-bearing 

 adambulacral, and below a ventral plate ; in this way the ambu- 

 lacral groove is caused to disappear, as it does from all but the 

 most primitive of Ophiuroids. The calycinal plates on the disk are 

 sometimes very distinct, sometimes quite obscure : the most pro- 

 minent structures are two shields at the bases of the arm, which, in 

 the light of subsequent morphological terminology, are seen to bear 

 unfortunate names ; they are called the radial shields. There is 

 often a strong plate at the side of the bursal cleft which is known 

 as the genital plate. On the oral surface of the disk we have the 

 mouth with its five slits ; the margin of the mouth is formed by 

 the swinging of the adoral ossicles to right or left and subsequent 

 fusion with half of the neighbouring arm. The spines at the sides 

 of the clefts are called the mouth-papilla? ; those at the angle, on 

 the plane of the surface of the mouth, tooth-papillae, and the larger, 

 stouter, and less numerous spines which form vertical rows within 

 are called teeth. In each ray, distal to these papillae, there is a 

 small round mouth-shield. The podia make their way to the 

 exterior between the arm-ossicles, and one or more small plates 

 lying close to the opening are called tentacle-scales. The Brittle- 

 star has no pedicellariae. 



Some of the Ophiuroids exhibit remarkable differences from 

 Ophiothrix ; the surfaces of the arm-ossicles form merely bosses and 

 depressions, and the two opposed surfaces move easily on one 

 another in various directions ; or the surfaces are distinctly saddle- 

 shaped. In the latter the arms may divide a few times, or many, 

 so as to give rise to that tangled-looking mass which is popularly 

 known as the Gorgon's Head or the Basket-fish (Astrophyton). 



In the regular Echinoidea the skeleton is very definitely and 

 simply arranged ; of the plates of the calycinal area, one set of 

 radial and one of interradial plates persists, but the centrodorsal is 

 absorbed and the digestive tract ends in the space which it filled. 

 In the general description of an Echinus, it is usual to speak of the 

 radial plates as the " oculars," the interradial as the " genitals '"' ; 

 this, because the former has been supposed to bear an eye-like 

 tentacle, and the latter have become connected with the fused and 

 secondarily interradially placed gonads which open to the exterior 

 by a duct which ends in these plates. Except in Cidarids and 

 Echinothurids this area is, however, always small ; the greater part 

 of the test is formed by what is known as the corona. In all recent 

 forms this always consists of twenty rows, arranged in ten pairs, 

 five of which are radial and five interradial in position. The radial 

 plates overlie the ambulacral or water-vascular tubes, and the podia 

 here make their way to the exterior through paired pores which 



