ORAL AND APICAL SYSTEMS OP THE ECHlNODBRMS. 379 



This extension of the extreme distal portion of the aboral 

 coelom throughout the rudimentary stem of the young 



Fig. XI. — Section through a Comatula larva. (After Gotte.) al. Diges- 

 tive sac. Ip. Left peritoneal space. rp. Right peritoneal space, 

 (r/j'). Its posterior diverticulum. st. Stem. wv. Rudiment of 

 water-vascular ring. 



Pentacrinoid is of great interest, for it is at the end of 

 the stem, i.e., at the apex of the aboral coelom, that the 

 terminal discoidal plate is developed ; and it therefore 

 appears to me to be strictly homologous to the subanal 

 plate of Toxopiieustes , which also appears at the centre of 

 the posterior end of the right peritoneal tube. 



This terminal plate of the stem of the Pentacrinoid is 

 not developed in the form of a ring like the true joints of 

 the stem, nor does any of that peculiar fasciculated tissue 

 appear in connection with it which forms the principal part 

 of the older stem segments (figs, vui and ix). But the base 

 of the sheaf of rods passing through the last ring of the 

 series abuts against the centre of its upper surface, and it 

 increases both in diameter and in thickness, to form the 

 skeleton of the round fleshy disc by which the stem adheres 

 to its point of attachment. Hence it is merely a temporary 

 structure, not forming any part of the calyx of the adult 

 Comatula. But I do not think that this point can be urged 

 as any serious objection to the view advanced above, that it 

 is homologous with the central plate of the apical system of 

 the other Echinoderms. In most Echinids and Asterids this 

 plate undergoes more or less subdivision and resorption. In 

 Collyrites there is scarcely any trace of it, and it is entirely 

 absent in the cretaceous Ananchytidcs, in the Spatangida, and 

 in Micraster} Further, in some of the recent Echini (Brissop- 

 sis, Meoma) it moves away from its central position out of 

 the calyx altogether, between the two rows of the last plates 

 of the odd interambulacrum. This is quite analogous to the 

 change of position which 1 suppose it to undergo in the 

 young Crinoid, though on a smaller scale. 

 ^ Loven, loc. cit., p. 83. 



