MUSEUM OF COMPARATIVE ZOOLOGY. 189 



median face of the respiratory cavity, and the lumen of the infolding 

 connects by a narrow orifice with the respiratory chamber (Fig. 88, 

 cav. mt.). I believe this is the organ first described by Lacaze- 

 Duthiers. 



A little farther forward the right visceral ganglion sends to the right 

 side of the body a nerve (Plate VII. Fig. 89 n.), which passes between 

 the wall of the mantle chamber and the primitive sexual duct, probably 

 to be distributed to the right half of the mantle. 



At this time the greater portion of the abdominal ganglion (Plate VII. 

 Figs. 81, 82, 85, 86, 93) lies on the right side of the median plane, al- 

 though it is joined to the left visceral by a large and prominent connect- 

 ive (Plate III. Figs. 50, 52, Plate V. Figs. 65, 66, Plate VII. Fig. 93). 

 Since the visceral ganglia have grown inward and forward, the ab- 

 dominal ganglion now occupies a position considerably posterior to 

 them (Plate VII. Figs. 83, 86) ; it lies above the right side of the radula 

 sac. Its posterior dorsal margin is still continuous with the ectoderm 

 of the wall of the respiratory cavity (Fig. 93), but farther forward it is 

 entirely separated from the ectoderm (Fig. 85), and is surrounded by a 

 layer of connective-tissue cells. All the other ganglia are similarly en- 

 veloped in connective tissue except where they are continuous with the 

 ectoderm. 



The connective to the left visceral ganglion (Plate VII. Fig. 93) passes 

 downward, forward, and outward to the left side above the radula sac. 



The buccal ganglia (Plate VII. Fig. 81) are larger than on the tenth 

 day, but are closely applied, as before, to the walls of the radula sac. 

 Their commissure (Plate V. Fig. 65) is embraced in the angle between 

 the oesophagus and the neck of the radula sac, and in sagittal sections 

 presents a circular outline. 



On the fourteenth day the upper lips as well as both pairs of tentacles 

 are very prominent, and the foot gland has grown backward still farther 

 into the foot (Plate VIII. Fig. 102). The salivary glands have now be- 

 come elongated into tubular organs with a circular lumen and thick 

 walls consisting of a single layer of epithelial cells (Plate VIII. Fig. 106). 

 They reach a little farther back than the buccal commissure ; in passing 

 forward they lie on either side of the oesophagus, about on a level with 

 its lower border. They pass along the dorsal side of the buccal ganglia, 

 and then suddenly bend downward to open into the oesophagus. 



The cerebral invaginations (Plate VIII. Fig. 96) present the same gen- 

 eral appearance as in the stage last described, but the lumen of the sacs 



