188 BULLETIN OF THE 



The connectives, both to the pedal (Plate VII. Fig. 81) and to the 

 pleural ganglia (Plate III. Fig. 48), are well developed, and are both 

 thicker and shorter than in the stage last described. 



The pedal ganglia do not differ materially from the condition de- 

 scribed for the eleventh day. The anterior end has increased in diam- 

 eter, and has grown a little farther forward (Plate III. Fig. 50, Plate 

 VII. Figs. 81, 91). 



Both commissures are now present ; the anterior (Fig. 92) is a little 

 behind the otocysts (compare Fig. 92 with Fig. 91), and the posterior 

 (Fig. 90) is directly above the blind end of the foot gland, and about 

 0.2 mm. back of the anterior commissure. 



The pleural ganglia (Plate III. Fig. 48, Plate VII. Figs. 82, 83, 88) 

 are very near the cerebral ganglia, as may readily be seen in sagittal 

 sections (Figs. 48, 82), and the fibrous connectives to the other ganglia 

 are plainly to be distinguished. The ganglia have become more com- 

 jjact and rounded, and occupy a position nearer the middle plane of the 

 body (Figs. 86, 88). 



The visceral ganglia (Plate III. Fig. 49 ; Plate VII. Figs. 83, 84, 

 86-89), although they have increased greatly m size, are still connected 

 with the ectoderm which forms the anterior wall of the mantle chamber 

 (Figs. 88, 89). 



They have also moved inward and forward. The right ganglion 

 (Figs. 49, 83, 87-89) is especially well developed, and much farther 

 forward than in the previous stage. Its axis is prolonged into a nerve, 

 which runs upward and backward, probably to the olfactory ganglion 

 (Figs. 84, 87). 



The connective from the right visceral to the abdominal ganglion 

 passes backward and inward (Plate VII. Figs. 83, 84). Where the 

 connective leaves the visceral ganglion (Fig. 83), the nuclei of the 

 ganglionic cells are very large, and the fibres are very much elongated 

 in the direction of the connective. 



In specimens cut crosswise the nerve which forms the dorsal prolon- 

 gation of the axis of the visceral ganglion is found far forward, in front 

 of the anterior face of the abdominal ganglion ; it passes upward and 

 inward (Plate VII. Figs. 87, 88), and is connected with the ectoderm 

 that forms the wall of the small infolding from the respiratory cavity 

 (Fig. 88) referred to in the account of the tenth day. This region is at 

 the same level as that with which the abdominal ganglion is connected 

 farther back (Plate VII. Fig. 93). The ectodermic cells to which this 

 nerve is distributed form the lining to an irregular infolding from the 



