No. I.] THE EMBRYOLOGY OF CREPIDULA. 29 



it traverses the cells of the transverse arms from tip to base. 

 The posterior branch, which is never functional, surrounds the 

 first quartette. When first formed the velar cells are not 

 ciliated, and they lie at the same level as the surrounding cells. 

 Later they are raised into a well-marked ridge, and are finally 

 drawn out into a very extensive wheel-shaped lobe, the long 

 velar cilia being borne around its margin. 



The shell gland appears on the postero-dorsal surface just 

 dorsal to the growing-point. It arises as a prominence of ecto- 

 derm cells, which from their position seem to be derived from 

 the posterior member of the second quartette. In the place of 

 this prominence an invagination afterward appears ; the margins 

 of the invagination extend rapidly, and a thin cuticle, the first 

 indication of the shell, is secreted by the invaginated cells. As 

 development proceeds the shell becomes asymmetrical, devel- 

 oping more rapidly on the left side than on the right. This 

 asymmetry extends to all the organs posterior to the foot and 

 head vesicle. 



The foot arises as a single median protuberance on the ven- 

 tral side of the body just posterior to the mouth, and in front 

 of the anal or growing region. In later stages the foot becomes 

 more and more prominent posteriorly, until it turns forward 

 and lies ventral to the mouth, though still attached to the body 

 posterior to the mouth. 



At the posterior end of the embryo three or four large ciliated 

 anal cells appear very near the growing-point, and at this place the 

 distal end of the intestine is in contact with the ectoderm. The 

 proctodeal invagination does not occur until late in develoioment. 



Later Changes. — The intestine is a tube with a distinct 

 lumen, its walls being formed of small cells free from yolk. 

 Its posterior end is formed first, and it grows in length chiefly 

 by the addition of cells at its anterior end, where it opens into 

 the space between the yolk cells. In the course of develop- 

 ment, the distal end of the intestine is carried forward on the 

 ventral side, and at the same time the whole hinder portion of 

 the embryo undergoes laeotropic torsion. By the continuance 

 of these two movements the distal end comes to lie in front of 

 the central end, and the latter is found successively on the right 



