1 36 CONK LIN. [Vol. XIII. 



around the margin of the velar lobe on the posterior side, being 

 separated from the preoral ridge by a shallow ciliated groove. 

 On the ventral surface these ridges are widely separated, the 

 preoral lying some distance in front of the mouth, as the post- 

 oral is some distance behind it. As shown in Figs. 81, 82, 

 the postoral ridge crosses the foot posterior to the large cili- 

 ated cells which lie just behind the mouth, and on the mid line 

 the ridge from each side turns backward and ends in a median 

 row of ciliated cells. The whole area between the anterior and 

 posterior ridges is clothed with a coat of fine cilia. 



Laterally the postoral ridge grows less prominent, and in 

 sections of the velar lobes taken about halfway between the 

 dorsal and ventral surfaces, the postoral velum is merely a 

 series of columnar, ciliated cells running around the posterior 

 margin of the velar lobes. Figs. 103-105. 



Passing up toward the dorsal side, the velum divides on each 

 side of the embryo into two branches. Figs. jZ, 80. The pos- 

 terior branch, which is much smaller than the anterior one, 

 continues up over the dorsal surface posterior to the head vesi- 

 cle, being incomplete, however, on the dorsal mid line. The 

 anterior and larger branch turns forward in a sharp curve on 

 each side of the body, and ends abruptly on each side of the 

 apex, Figs. 80, 82. The cells lying between the two branches 

 on the dorsal surface are the large ciliated cells of the posterior 

 cell plate. 



At the point where the branching occurs, one large cell is 

 found directly in the angle between the two branches. Figs. 

 77-80. This is, I believe, the posterior turret cell of each 

 side. I could not determine whether the turrets contribute 

 anything to the formation of the velum on the dorsal side as 

 they do on the ventral. In one sense they lie within the 

 velum, as does the whole posterior cell plate, being bounded 

 in front by the anterior and behind by the posterior branches, 

 but in any case the foremost row of the velum lies nearer the 

 apex on the dorsal side than it does on the ventral, since it 

 runs on the apical side of the posterior turret cells. From its 

 position relative to the apical organ and the large cells of the 

 posterior plate, it is probable that this anterior branch of the 



