I'JO 



BULLETIN OF THE 



is smaller (Plate X. Figs. 121, 12G) ; in cross sections (Fig. D) it ap- 

 pears oval. The walls are thick, being composed of spindle-shaped cells 

 arranged perpendicularly to the axis of the sac and so crowded that the 

 nuclei are three or four deep. 



The proliferated portion of the cerebral ganglia (Plate IX. Fig. 114) re- 

 tains its pear-shaped condition, but is shorter and thicker. A ventral and 



Figure D. — Posterior face of a transverse section from an embryo of the fourteenth day. X 190. 



cnch. Shell gland. 



cr. Heart. 



gn. ceb. Cerebral ganglion. 



hp. Liver. 



hp. dx. Right lobe of liver. 



in. Intestine. 



iv. ceb Cerebral invagination. 



mt. Mantle. 



Nephridial organ. 



aSsophagus. 



Pericardium. 



Pleural ganglion. 

 sac. rad Radula sac. 

 ia. dx. Right ocular tentacle. 

 ta'. Labial tentacle. 



vsc. Visceral ganglion. 



nph. 

 a'. 

 pi. cr 



pin. 



median poi-tion of each ganglion forms a small rounded lobe (Figure E). 

 These lobes are near the bases of the upf)er lips, and in sagittal sections 

 appear almost completely separated from the larger part of the ganglia 

 by ingrowths of connective tissue. It is from these lobes that the pedal 

 connectives arise. The connectives to the pleural ganglia emerge from 



