8 ECHINODERMA. 
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-papille; these at the angle, on 
the piane of the surface of the mouth, tooth-papille, and the larger, 
stouter, and less numerous spines which form vertical rows within 
are called teeth. In each ray, distal to these papilla, 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 pedicellari. 
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 
