MUSEUM OF COMPARATIVE ZOOLOGY. 193 



left visceral, and also nearer to the median plane (Plate VII. Fig. 95, 

 Plate VIII. Fig. 102, Plate IX. Fig. 120). It is in contact with the 

 lower surface of the right end of the abdominal ganglion (Plate VII. 

 Fig. 95). 



The abdominal ganglion (Plate VII. Fig. 95, Plate VIII. Figs. 101, 

 102, 104, Plate IX. Figs. 115-117, Plate X. Fig. 123) is entirely un- 

 connected with the ectoderm, and has moved forward, so that there is a 

 considerable space between it and the pleural groove, but its posterior 

 face extends farther back than that of the right visceral ganglion (Plate 

 VIII. Fig. 102). The greater portion of the ganglion is now situated 

 on the right side of the body, immediately above and to the right of the 

 radula sac (Plate VIII. Fig. 104, Plate IX. Figs. 115-117). It is 

 elongated, and its chief axis is directed obliquely across the body, the 

 right end being considerably higher and a little farther back than the 

 left end. In passing downward and forward to the left side of the body, 

 it lies between the oesophagus aud the posterior part of the radula sac. 

 Its left end is prolonged into a connective, which passes forward and 

 outward to join the left visceral ganglion (Plate VIII. Fig. 104, Plate X. 

 Fig. 123). A large nerve, which passes upward and backward to be dis- 

 tributed to the viscera, emerges from the most dorsal portion of the 

 abdominal ganglion on the right side of the body (Plate VIII. Fig. 104, 

 Plate IX. Fig. 117). The histological condition of the abdominal gan- 

 glion is similar to that of the previously described ganglia of this stage. 

 The fibrous portion, as well as the enlarged cells and nuclei, are espe- 

 cially prominent in the portion of the ganglion which lies to the right of 

 the median plane of the body (Plate IX. Fig 117). 



The buccal ganglia (Plate VII. Fig. 95, Plate VIII. Figs. 102, 106, 

 Plate IX. Fig. 120, Plate X. Fig. 121) have become larger, and with 

 their commissure (Plate VIII. Fig. 101, Plate IX. Fig. 120) stretch 

 across the dorsal wall of the neck of the radula sac, to which they are 

 still closely united. The nuclei immediately surrounding the central 

 fibrous portion of the ganglion are already slightly enlarged, though the 

 cells are not so far advanced in their histological differentiation as are 

 those of the other ganglia. A single pair of connectives passes obliquely 

 forward, downward, and outward, to join the buccal to the cerebral 

 ganglia (Plate X. Fig. 121). 



By the sixteenth and seventeenth days, besides a general increase in size 

 of the external organs, the foot gland extends backward much farther 

 vol. xx. — no. 7. 13 



