MONOGRAPH OF THE EXISTING CRINOIDS. 



455 



On the second day the edges of the vestibular invagination approach each 

 other and finally grow together in the posterior portion, this union of the two sides 

 gradually extending anteriorly until an extensive tubular structure is formed which 

 opens to the exterior on the ventral side through a constantly decreasing opening 

 behind the second ciliated band. 



The vestibule never appears to become entirely closed during the free-swim- 

 ming period, but onl^y after fixation. 



The vestibular tube later extends under the fourth ciliated band, but whether 

 this is an extension of the open furrow or of the closed tube itself has not been 

 determined. If concerned at all, the fourth ring can only play a small part in 

 the formation of the vestibule. 



Occasionally, in addition to the normal closure from the posterior end, the 

 sides of the anterior portion also grow together, though this only takes place to 

 a limited degree. 



In the still open anterior portion the vestibule shows in section essentially 

 the same features as in the younger larvae, except that it has become somewhat 

 deeper and has increased in size proportionate with the increase in size of the 

 entire larva. 



Farther posteriorly, where the closure is already completed, the vestibule 

 appears tubular, strongly compressed dorso-ventrally. The lumen is very narrow, 

 the walls very thick, especially dorso-laterally ; they are thinnest along the floor 

 of the groove. 



At the end of the free-swimming larval period the ciliation of the vestibular 

 invagination disappears. The cilia appear to persist longest in the anterior por- 

 tion in two narrow lateral zones running near the border in which the cells are 

 richer in plasma than elsewhere. In these zones the cell borders can not be 

 differentiated throughout. The nuclei lie in the cell end toward the body cavity, 

 whicli is richer in plasma than the other, while outwardly the plasma appears 

 netlike, inclosing a homogeneous substance. 



In preparations treated with absolute alcohol the intermediate substance 

 retains ha;matoxylin stain to a remarkable degree, and in such sections it appears 

 in the form of intensively colored club-shaped figures. 



In the portion of the vestibule already closed into a tube the wall is essentially 

 single layered. The lateral cells are of considerable length and carry the nuclei 

 at different heights. Even when the cilia have disappeared, the cuticle-like cell 

 border can be demonstrated. 



Intesttn-al Canal. 



Of all the internal organs the intestinal canal undergoes probably the least 

 change during the later embryonal and free-swimming period. 



As last described it was an entirely isolated sacklike structure surrounded by 

 the two ccelome pockets and the beginnings of the water vessel. At no place does 

 it come in contact with the ectodermal body wall, from which it is entirely excluded 

 by the surrounding organs. 



142140— 21— Bull. 82 31 



