TETHYS DACTYLOMELA (RANG) 2 5 



phore, giving off a few delicate branches to the integument near 

 its base, and terminating in a small ganglion at its distal extremity. 



The fourth, or optic nerve (4), is very slender and quite 

 long. It arises immediately behind and exterior to the third, 

 and is inclosed in a common sheath of connective tissue with it 

 for a very short distance, much less than that described by Maz- 

 zarelli for the Mediterranean species. It is unbranched, and 

 passes directly to the eye. No trace of an optic ganglion can be 

 made out at its base with the dissecting microscope, though sec- 

 tions might show its presence. 



The fifth nerve (^5), arises from the lateral border, imme- 

 diately in front of the cerebro-buccal connectives and is rather 

 slender. On the left side it courses forward and is distributed 

 to the body wall in the region of the mouth. Its fellow of the 

 right side passes forward, and divides into two branches, one of 

 which is soon lost among the muscles of the body wall, imme- 

 diately below the anterior portion of the penis. The second, 

 more slender branch courses forward, parallel to the penis, giving 

 off three branches at intervals, which pass to the latter organ, the 

 main nerve being finally distributed to the muscles of the mouth 

 region. 



The sixth pair form the acoustic nerves. They are included 

 in the cerebro-pedal connectives for about one-half of their length, 

 and then become separate from them as very delicate nerves, 

 passing directly to the otocysts, rounded capsules lying close to 

 the bases of the cerebro-pedal connectives upon each side. These 

 nerves are not shown on PI. II. 



Pedal ganglia. The pedal ganglia (ped. g.), are the largest 

 of the central nervous system, 3.0 mm. in greatest diameter, 

 approximately circular in general outline, flattened upon the 

 antero-ventral surface and strongly arched upon the postero- 

 dorsal face, thus having a nearly hemispherical form. They are 

 united below the esophagus by a stout, transverse commissure, 

 (p. com.), 3.0 mm. in length, and i.o mm. in diameter. This is 

 inclosed within a broad, flattened sheath of connective tissue, 

 which also contains near its upper anterior margin a very delicate 

 subcerebral commissure, connecting the cerebral ganglia together 

 below the esophagus. This latter commissure is so fine that it 

 is made out with certainty only in transverse, serial sections of 



