EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 391 



Altogether stage E gives us some great advances upon D. The formation of the 

 adult entity is marked chiefly — (1) by the general squeezing together and consolidation 

 of the mass of organs, so that their future relations are fixed ; (2) by the rapidly 

 developing pentamerous axial symmetry of the hydrocoele from one to five, followed by 

 that of the left posterior coelom ; (3) by the curving of the larval mesenteron in a vertical 

 (larval) plane, and the complementary flattening of the contiguous surfaces of mesenteron 

 and right posterior coelom. These internal changes are sufficiently great to throw the 

 balance over towards the adult side, and from this onwards it is clear that we are dealing 

 with a starfish, and not with a bilateral animal. Hence it is only right that the 

 nomenclature should be changed as far as may be. The term ' pre-oral ccelom ' may still 

 be retained, but the following changes are already sufficiently clear : 



Stage D {Bilateral). Stage E {Axial). 



Left lateral coelom = Hydrocoele. 



Right „ „ = Epigastric ccelom (Goto). (Aboral coelom.) 



Left posterior ccelom | , , TT , . , /r . . 



_. . . , > fused = Hypogastric ccelom (Goto). 



Right posterior ccelom ) 



Central ccelom = Dorsal sac (Bury). 



Right side = Aboral surface. 



Left side = Oral surface. 



We can observe that the hydroccele passes from a circular disc to a rayed disc, 

 and does not form a ring till later by perforation of its centre ; further, the radius 3 is 

 exactly posterior, or as nearly so as it is possible to identify. The hypogastric ccelom, 

 however, is from the outset curved : in fig. 52 it is already seen in early D (cf. fig. 

 57) to be curved into a half-circle, with its dorsal and ventral horns. In stage E its 

 ventral horn evidently lies over radii 1 and 2, and is continued forwards and upwards 

 till it is just at the base of 5, though it is shut out from the dorsal surface of this radius 

 by the still persistent pre-oral ccelom. The dorsal horn extends round till it lies at the 

 base of radius 4, beyond which it comes in contact with the pre-oral ccelom. Hence 

 radii 4 and 5 are not yet completely covered by their share of the hypogastric ccelom. 

 The hypogastric ccelom is enabled to extend as far round as this owing to the contracting 

 of the pre-oral ccelom in the ' neck ' region (see diagram 2, p. 400). 



If we number the radii of the hypogastric ccelom, we see that the most anterior one 

 lies at the base of hydroccelic radius 5, the next two ventral, II. and III., are over 

 radii 1 and 2 respectively, IV. is over radius 3, and the last, i.e. V., is very nearly 

 over radius 4 of the hydroccele. Thus we already have the curious twisting phenomenon 

 of the oral or hydrocoele element upon the aboral or ccelomic part which has been 

 described for Asterina by Ludwig and for several other asterids, though said to be absent 

 in Bipinnaria asterigera by Bury. In most cases, however, this twist takes place at a 

 later stage, when its progress can be actually observed externally. 



Stage F. — We may now pass to stage F, as shown in figs. 90 and 91. Here a 



