628 R. J. HARVEY GIBSON ON THE 



two anterior and the two posterior ganglia to each other lie between the 

 pharynx and the muscles of the infraradular sheet. Tracing the loop above 

 mentioned round the cerebral commissure, it is found to bend suddenly back- 

 ward, and to unite itself with a small ganglion on a second cerebral commissure 

 which lies beneath the buccal mass. These two small ganglia on this second 

 commissure have been considered as inferior buccal ganglia ; and, since 

 the nerves from them supply the ventral protractors of the odontophore, they 

 may be so named. 



From the posterior part of either cerebral ganglion two slender white 

 commissures pass backwards along the sides of the neck — the exterior to join 

 the visceral ganglion, the interior to join the pedal ganglion. The pedal and 

 visceral ganglia form one thick hoop of nerve matter rather than four ganglia 

 unitedly commissures. The visceral ganglia are, as already stated, united 

 to the cerebral ganglia by slender commissures, to the pedal by a thick short 

 band. They give off two important nerves on either side of the body, — first, 

 externally, the musculo-pallial nerve (which soon splits into two branches, 

 which go to supply the branchiae and mantle and the circular muscle 

 respectively) ; and, internally, the splanchnic nerve. This splanchnic, soon 

 after it leaves the ganglion, gives off a delicate nerve which travels back along 

 the pharynx, and may be termed the recurrent nerve. The left splanchnic 

 itself mounts the left shoulder, passes beneath the salivary glands, and, cross- 

 ing the right splanchnic, gives off a minute branch to the right ctenidium; i.e., 

 the right ctenidium is supplied by the left splanchnic. The nerve then 

 crosses back and unites on the way with the right splanchnic. The combined 

 nerve gives off a number of branches to the viscera as it goes ; and finally, 

 crossing over to the left side once more, it supplies the left ctenidium. (We 

 are indebted to Spengel for this discovery, and for the important suggestion that 

 accompanies it, viz., that these cteniclia are really the rudiments of the lost true 

 gills.) Spengel describes a minute ganglion, the olfactory ganglion lying near 

 the ctenidia ; that the writer has not been able, however, to see. 



The pedal ganglia lie between the two visceral ganglia, close together and 

 united by a very thick and short commisssure. They are united, as already 

 stated, to the cerebral ganglia by long slender commissures, one for either 

 ganglion. They give off into a slit in the muscle of the foot two large nerves 

 each, one of which supplies the deep muscles of the foot, the other the super- 

 ficial muscles. 



Both the pedal and musculo-pallial nerves divide ultimately into a large 

 number of secondary branches. 



The histological structure of the nervous system is extremely simple. The 

 nerves are composed of fibres, each of which is a very elongated, nucleated, cell. 

 The cell itself is band-shaped and fibrillated, nucleus oblong, taking on a deep 



