142 CONK LIN. [Vol. XIII. 



in later stages it is much broader than long. On the median 

 surface of the foot there are several large ciliated cells which 

 resemble the apical and dorsal cells, and which I have there- 

 fore called the pedal cell plate. All the rest of the foot is 

 covered by a columnar epithelium of ectoderm cells. When it 

 first appears it is about equally prominent over its whole surface, 

 Figs. ^6, JJ, but in the course of further development the pos- 

 terior part becomes much more prominent than the anterior. 

 The foot is, as it were, tipped up on its anterior edge by being 

 crowded forward from behind. This forward tilting continues, 

 as shown in Figs. 80-82, until the foot, instead of lying pos- 

 terior to the mouth as it did at first, lies ventral to it. At an 

 early stage the ectoderm forming the foot separates from the 

 yolk beneath, and the cavity thus formed becomes traversed in 

 every direction by mesoderm cells. 



About the time that the supraoesophageal ganglia first appear 

 the otocysts arise as small invaginations of the ectoderm on 

 each side of the foot. They are at first open pits, which 

 gradually close, forming vesicles the outer walls of which 

 lie in the layer of ectoderm covering the foot, Fig. 100. At 

 iirst the vesicle is quite small, and the cells surrounding it are 

 cuboidal, but in later stages it increases in size and its walls 

 grow thinner, Figs. 80-82, 105 ; at the same time its outer 

 wall separates entirely from the ectoderm covering the foot 

 and the vesicle comes to lie entirely within the cavity of the 

 foot. The cerebro-pedal connectives end directly against the 

 otocysts, and a small strand of cells, the origin of which I 

 have not determined, connects the otocysts of the two sides. 



In the oldest embryo figured the foot bears a thin cuticular 

 operculum over its posterior surface. At a still later stage 

 it becomes much more prominent and is triangular in outline, 

 the apex being directed ventralward and forward. A very great 

 number of cells which stain deeply and are probably gland 

 cells are distributed quite uniformly in its epithelium. 



As has been mentioned (p. 131), Lillie has found that the 

 foot in Unio is formed from a portion of the ventral plate, 

 which is derived from the first somatoblast, X (2d). I have 

 called attention to the fact that in Crepidula the cell rows 



