viii ECHINODERMATA ONTOGENY f>41 



The stone canal breaks through into the parietal sinus. The point at which 

 this occurs does not, however, correspond with the point at which the hydroccel and 

 the parietal sinus originally were in open communication. 



The parietal sinus also takes part in the shifting just mentioned. In the free- 

 swimming larva it lay in front of the hydroca-1. This position it retains, while 

 shifting backward (towards the oral end) together with the hydroccel. It thus 

 approaches its external pore. Compared with the other growing organs, it remains 

 small and stationary. 



The hydrocoel is thus connected by the stone canal with the parietal sinus, and 

 this latter is in open communication with the exterior through the hydropore. 



The intestine. An extraordinary process goes on in the intestinal vesicle. 

 Numerous cells become detached from its wall, and wander into its cavity, which 

 they finally completely fill. They fuse for the most part into a large yolk-like mass, 

 which is entirely resorbed at a later stage as nutritive material. 



The floor of the vestibule deepens at the centre, and is produced anteriorly like a 

 funnel towards the intestinal vesicle. This funnel, which passes through the hydro- 

 ccel ring, becomes the oesophagus, and joins a posterior process of the intestinal 

 vesicle which grows out to meet it. 



The intestinal vesicle divides into a stomachal section to the left of the larva 

 and a narrower portion running dorsoventrally on the right side. This latter part, 

 the hind-gut, rises with a broad base out of the former and ends blindly. The blind 

 end of the hind-gut then grows over to the left ventrally. 



The ccelom. The two ccelom sacs shift and spread out in a peculiar manner. 

 The left sac shifts quite posteriorly, and becomes the oral ccelom, which grows round 

 the oesophagus on all sides from above downward, thus assuming the shape of a 

 hollow horse-shoe which clasps the oesophagus, the stone canal, and the parietal 

 canal (counting these in order from within outward). The gape of this horse-shoe 

 is directed ventrally to the left, and since the tips of its two arms grow towards one 

 another, a short longitudinal accessory mesentery arises. The right ccelom vesicle 

 passes through changes of form, expansion, and shifting which are difficult to describe 

 briefly, and becomes the aboral or apical ccelom. In consequence of the shiftin^s of 

 these vesicles the longitudinal principal mesentery which separated the originally 

 right from the originally left ccelom vesicle becomes a transverse mesentery, 

 separating the oral from the apical ccelom, and surrounding the oesophagus like a 

 diaphragm. Near the right (now aboral) ccelom also, a longitudinal, somewhat 

 diagonal accessory mesentery develops, which runs somewhat to the right of the 

 ventral median line. The walls of the apical (originally right) ccelom are continued 

 anteriorly into the walls of the five tubes which form the chambered sinus, but at 

 the point where they pass into one another they are so pressed together that no 

 open communication exists between the two sinuses. 



The axial organ (genital stolon) of the calyx arises as a thickening in the left 

 epithelial wall of the longitudinal accessory mesentery of the aboral ccelom, at its 

 most anterior (apical) end, where the chambered organ commences. As the genital 

 strands of the arms and pinnulae most probably arise as outgrowths of the axial 

 organ, it might thus be proved that in the Crinoids also the genital cells are derived 

 in the last instance from the endothelium of the body cavity. The cushion-like 

 thickening increases in length, becoming partly constricted from the mesentery ; 

 posteriorly, it reaches to the oral ccelom ; anteriorly, the axial organ passes into the 

 stalk up the centre, between the five tubes of the chambered sinus. 



The formation of trabeculse begins in the aboral ccelom. Single endothelial 

 cells lengthen and project like pillars into the ccelomic cavity. A similar process 

 takes place in the hydroccel. 



The skeleton. When the vestibule shifts to the posterior end of the larva the 



