No. I.] THE EMBRYOLOGY OF CREPIDULA. 135 



lobes which curve backward and downward on each side of the 

 mouth, Fig. 79. Because of these opposite movements of the 

 median and lateral portion of the preoral velum, but chiefly 

 through the backward and downward growth of the lateral por- 

 tions, the velum, when seen in apical view, bears a deep sinus 

 on the ventral mid line. Figs. 79, 82. 



In these later stages, Figs, "j^ et seg., the first velar row 

 forms a slight ridge across the ventral mid line just posterior to 

 the apical cell plate, which is particularly well marked because 

 composed of a single row of small cells with densely staining 

 nuclei which are bounded in front by the very large cells of 

 the apical plate and behind by the large cells of the second 

 velar row, Figs. 78, 79. Behind the cells of the second row 

 and on the very edge of the mouth-opening is a third well- 

 marked row, consisting like the first of small cells with densely 

 staining nuclei. Figs. 79, 81, 82. This row can be traced lat- 

 erally to the place where it joins the first row to form the 

 margin of the velar lobe. At this point it bears a pair of 

 prominences which ultimately become the tentacles. Fig. 81, 

 T ; these structures therefore are formed in the preoral velum, 

 in fact in the second cell row of the prototroch. In later 

 stages they lie over the cerebral ganglia. 



On the mid line these rows of velar cells are raised but 

 a little above the general level, but laterally they are borne 

 on the margins of the very prominent velar lobes. Cross 

 sections of these lobes show one row of large rounded cells, 

 which forms the extreme margin of the lobe and bears the 

 long velar flagellae. On each side of this are one or more 

 rows of large crescentic-supporting cells, Figs. 103-105. 

 Similar cells have been described by Patten ('86) as present in 

 Patella. 



The postoral velum is not well defined until the last stage 

 shown in the drawings. Figs. 81, 82, though somewhat irregu- 

 lar rows of nuclei can be seen crossing the body posterior to 

 the mouth in stages as early as Fig. ^6. In these later stages 

 a ridge of cells runs out from the posterior edge of the velar 

 lobe and can be distinctly followed to the ventral mid line of 

 the foot. This is the postoral ridge of velar cells, and it runs 



