192 BULLETIN OF THE 



terior ends are elongated and continued as two large nerves far back into 

 the foot. In specimens cut crosswise these nerves appear as rounded 

 patches of fibres, situated one on each side of the body, above the plane 

 of the foot gland and about midway between it and the lateral walls of 

 the foot. Each is surrounded by a layer of connective-tissue cells. As 

 one approaches the pedal ganglia in passing from behind forward, the 

 nerves increase in size and lie nearer to each other. In the region of 

 the posterior commissure (Plate IX. Fig. 119) the ganglia are nearly as 

 broad as in the region of the anterior commissure (Fig. 118), but they 

 are not much more than half as thick in the dorso-ventral direction. In 

 front of the posterior commissure they are separated by a narrow space, 

 which is wider behind than in front, where it is terminated by the ante- 

 rior commissure. The commissures are both well developed (Plate VIII. 

 Figs. 101, 102, Plate IX. Figs. 118, 119), and owing to the approxima- 

 tion of the ganglia have become shorter than in the last stage. The 

 nuclei in the region of the posterior commissure (Fig. 119) are of nearly 

 uniform size; but in front of it each ganglion (Figs. 114, 118) contains 

 a fibrous central portion immediately surrounded by the greatly enlarged 

 nuclei of cells which form the most of the fibrous substance. 



The pleural ganglia (Plate VIII. Fig. 106, Plate IX. Figs. 114, 116, 

 Plate X. Figs. 123, 125, and Fig. E) have increased considerably in size, 

 and are more compact. They have moved downward and inward ; and 

 each now lies in contact with the posterior face of the corresponding 

 cerebral mass (Plate IX. Fig. 114), and below and in front of the ven- 

 tral portion of the corresponding visceral ganglion (Figs. 106, 123, 

 125). They are much smaller than either the cerebral or visceral 

 ganglia. The nuclei of their central cells are, as in the pedal ganglia, 

 much enlarged. 



The visceral ganglia (Plate VII. Fig. 95, Plate IX. Fig. 114, Plate 

 X. Figs. 123, 125) are now entirely detached from the ectoderm, and 

 have moved downward, forward, and inward. 



The left ganglion (Plate VIII. Fig. 106, Plate X. Fig. 125) is smaller 

 than the right, and more closely connected with the left pleural (Fig. 

 125) than in the previous stage. Its dorsal surface is slightly above 

 the level of the dorsal wall of the radula sac, and its connective with 

 the abdominal ganglion (Plate VIII. Fig. 104) is much broader than 

 before. The right visceral ganglion (Plate VII. Fig. 95, Plate VIII. 

 Figs. 102 and 106, Plate IX. Fig. 114, Plate X. Fig. 123) is much 

 larger than in the last stage ; it is also closely connected with the right 

 pleural ganglion (Fig. 123). It extends dorsally much farther than the 



