MUSEUM OF COMPARATIVE ZOOLOGY. 183 



ectoclermic cells from the walls of the foot (Plate IV. Fig. 57, 58, prf.). 

 The first decided evidence of a pedal commissure makes its appearance 

 during this stage. It consists (Fig. 54) of a few very much elongated 

 nerve cells, which stretch across from one ganglion to the other a little 

 posterior to the region of the otocysts. The commissure may be traced 

 on about half a dozen successive sections, or for a distance of some 50 

 or 60 jx. From its position it evidently is the beginning of the anterior 

 commissure. The thickness (10 fx) of a single section contains only 

 three or four cells, the nuclei of which have the chromatic substance 

 so concentrated into a single nucleolus as to make the nuclei appear 

 clearer than those of the surrounding connective-tissue cells. There 

 is at present no trace of a posterior commissure. The otocysts are now 

 nearer the ganglia (Plate IV. Fig. 58, Plate V. Fig. 63) than at any 

 previous stage. 



The pleural ganglia (Plate V. Fig. 63) are still inconspicuous, being 

 composed of only a few scattered cells, which lie nearly dorsal to the 

 otocysts, about midway between the visceral and the cerebral ganglia 

 of the same side of the body. Many of the cells are elongated in the 

 direction of the ganglia between which they are located, and appear to 

 form the beginning of a connective between them. 



The visceral ganglia (Plate IV. Figs. 58 and 59, vsc.) are still con- 

 nected with the ectoderm, but project more prominently from the wall 

 of the body, and extend forward more than before. The right (Plate IV. 

 Fig. 59) is larger, and still lies more dorsal, than the left (Plate V. 

 Fig. 63). The cells which compose the ganglia are numerous and 

 large, and the nuclei of those which form the centre of the ganglion are 

 conspicuously larger than those at the periphery. In cross sections of 

 a stage possibly a little less developed than the one last described, the 

 ganglia (Plate IV. Figs. 53, 56, 57, 55) lie, one on each side of the 

 body, immediately above the pleural groove, a little below and inside 

 the external orifices of the primitive nephridial organs. On the right 

 side of the body the ectoderm which constitutes the anterior wall of 

 the infolding to form the mantle chamber is seen in sagittal sections 

 (Plate IV. Fig. 58) to be much thicker in the region adjoining the pleu- 

 ral groove than in that which forms the deeper portion of the infold- 

 ing. The transition from the thick to the thin ectoderm is very abrupt, 

 and is marked by a pocket-like depression. The right visceral ganglion 

 is situated at the side and in front of this depression. Some of the cells 

 in the anterior portion of this ganglion (Plate IV. Fig. 56) are traceable 

 toward the median plane of the body. The left visceral ganglion 



