ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 177 



mesentery, which is formed by the fusion of the walls of the 

 two peritoneal cseca, and supports the rectum.^ Very early, 

 however, there is a general forward movement of all the 

 ventrally placed structures, namely, the depression marking 

 the position of the future mouth, the water-vascular sac, the 

 oral coelom, and the blastopore. These all take up a new 

 position at the anterior end of the Crinoidal axis (transverse 

 axis of the Gastrula), and it is here that the ventral or 

 actinal surface of the future Comatula begins to appear, in a 

 plane very considerably inclined to that of the ventral sur- 

 face of the Gastrula. 



This rudimentary actinal surface primitively consists of 

 nothing but the area occupied by the oral plates, together 

 with the bases of the arms. But as the digestive tube 

 increases in length and acquires an anal orifice, the radials 

 spread out very much, so as to extend the base of the cup ; 

 while the single anal plate, originally interposed between 

 two of them, begins to be lifted out, as it were, from this 

 position, as it is attached more to the growing visceral mass 

 than to the neighbouring plates. 



At the same time the diameter of the oral circlet, still 

 embracing the ring of oral tentacles, continually decreases 

 in proportion to that of the disc or visceral mass, as the 

 size of the latter is increased by the development of the 

 alimentary canal. 



Finally, the oral circlet detaches itself from the summit 

 of the radials on which it previously rested, and is relatively 

 carried inwards by the great enlargement of the circle formed 

 by them. The space between the two series is now filled in 

 only by the membranous perisome of the above-mentioned 

 equatorial zone, over which the ambulacral grooves extend 

 themselves, passing outwards from the circum-oral region 

 between the oral valves towards the growing arms. 



The diameter of the visceral mass continually increases, 

 so that it extends nearly as far as the bifurcation of the 

 arms, and the oral circlet is thus separated by a much wider 

 interval from the periphery of the disc. It is in this outer 

 ring that the anal funnel is situated, the anal plate which 

 it bears on its outer side being altogether lifted out from 

 between the two radials which it originally separated. In 

 some Palceocrinoidea, however, it retains its primitive 

 position within the radial circlet. 



I have already mentioned^ that in most recent Crinoids 

 the orals of the larva are completely resorbed, so that no 



1 Gotte, loc. cit., pp. 591, 601, 609. 



2 Part I, p. 355. 



