34 



to the fusion of its lateral edges in tlie median line, a 

 small anterior aperture being retained. The intestine 

 assumes the form of a hollow plate, and the hydrocoel lies 

 in its ventrally directed cavity (fig. 65). The rudiments 

 of the chambered organ now appear as five tubular out- 

 growths from the right enterocoel vesicle ; and the skeletal 

 joints of the stalk, which are at first horse-shoe shaped, 

 surround them. The hydrocoel vesicle separates from the 

 parietal sinus and assumes a horse-shoe shape. Five 

 outgrowths of its Avail appear, each of which eventually 

 gives rise to three primary tentacles, while the primary 

 water tube (stone canal) appears at the blind end of the 

 left limb of the vesicle. The parietal sinus takes up a new 

 position in front of the hydrocoel and finally communicates 

 with the exterior through the hydropore. Soon after the 

 larva attaches itself the ciliated rings and tuft and the 

 neural plate disappear. The vestibular aperture dis- 

 appears and the vestibule itself takes up a new position at 

 the posterior end of the larva, and by the assumption of 

 a pentagonal form, determines the radiate structure of the 

 adult. It is accompanied in its movements by the 

 hydrocoel, so that the two structures maintain their relative 

 positions. Communication between the hydrocoel vesicle 

 and the parietal sinus is re-established by the water tube, 

 which breaks through into the former. Concurrently with 

 these changes a store of nutritive material is accumulated 

 within the intestinal vesicle by numerous cells which 

 become detached from its wall and finally completely fill 

 it. A funnel-like depression of the floor of the vestibule 

 passes through the hydrocoel ring and forms the 

 (Esophagus. A corresponding p?ocess of the intestinal 

 vesicle eventually fuses with it. The left ccelomic sac 

 becomes the oral coelom, and the right sac the aboral or 

 apical coelom. 



