viii ECHINODERMA TA — ONTOGEN Y 541 



The stone canal breaks through into the parietal sinus. — The })oint at wliieli 

 this occurs doew not, however, corresijond with the point at whicli the hydrociel and 

 the parietal sinus originally were in open comniiinication. 



The parietal sinus also takes part in the sliiftiiig just mentioned. In the I'ree- 

 swiniming larva it lay in fi'ont of the hydrocrel. This position it retains, while 

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

 approaches its external pore. Com[)ared with the other growing organs, it remains 

 small and stationarj\ 



The hydroccel 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 filh They fuse for the most part into a large yolk-like mass, 

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



Tlie floor of the vestibule deepens at the centre, and is i)roduced anteriorly like a 

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

 coel 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 lell of the larva 

 and a narrower portion running dorsoventrally on the right side. Tliis 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 coelom. — The two ccelom sacs shift and s])read out in a peculiar manner. 

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

 the (esophagus on all sides from above downward, thus assuming tlie shape of a 

 hollow horse-shoe which clasps the cesophagus, 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. Tlie right coelom vesicle 

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

 briefly, and becomes the aboral or apical ccelom. In conse(|uence of the shiftino-s of 

 these vesicles the longitudinal principal mesentery which separated the orio-inally 

 right from the originally left ccelom vesicle becomes a transverse mesentery 

 separating the oral from the apical cojlom, and surrounding tlie c(.\soj)liagus 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 pinnulaj 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 j^lace in the hydroccel. 



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



