The anatomy of Lottia gigantea Gray. 43 



and outward the meet them. Practically the whole of the pleuro- 

 pedal connective is ganglionic. The pedal cords pass directly back- 

 ward and outward in the cavity of the neural arteries and form with 

 the posterior commissure {Pc") a rude ellipse. Just back of the 

 anterior commissure (Pc) is a small and very distinct secondary com- 

 missure (Pc') uniting the two cords. Ganglion cells extend part way 

 across the posterior commissure {Pc"). Back of this connective the 

 pedal cords {Ped. G") are still ganglionic, and remain so till the 

 branching takes place. 



Three sets of nerves are given off from the pedal ganglia and 

 cords : 1) those innervating the foot proper, 2) small nerves supplying 

 the spindle muscle, 3) fine nerves given off on the inner side of the 

 cords to the central portion of the foot. The foot nerves {Ped.N) 

 are by far the largest, and they leave the pedal cords on the outer 

 and lower aspect. Usually there are about sixteen nerves to each 

 side, but the number varies, and an exact bilateral symmetry of number 

 is by no means constant. The nerves pass downward and outward in 

 a slanting plane, and very soon begin to branch, and anastomose with 

 those next to them, the net becoming finer and finer as the edge of 

 the foot is approached. Not only is there a branching in this single 

 plane but also, to a more limited extent, in dorsal and ventral di- 

 rections. Usually about half way from the pedal cord to the edge of 

 the foot rather prominent ventral branches depart from the main pedal 

 nerves, and these in turn ramify over the sole of the foot. So many 

 nerves end in the edge of the foot it is not surprising that the margin 

 is one of the most sensitive portions of the whole animal. In watching 

 the creatures crawl one sees at once that they depend largely on this 

 restless margin for orientation, and that as they move along it re- 

 sponds to any irregularity or roughness of the surface quite as quickly 

 as do the tentacles. The pedal cord beyond the posterior commissure 

 loses its ganglion cells and divides, just as do the ordinary foot nerves, 

 the branches anastomosing freely with those of the nearest pedal 

 nerves. In front, also, there is a continuous network in the anterior 

 portion of the foot, as indicated in Fig. 36. 



The nerves supplying the spindle muscle {Sh. M. N) are small and 

 are imbedded in the superficial muscles of the body wall. Like the 

 pedal nerves, they vary, apparently, ia number. The anterior most of 

 these nerves are the largest and they are also more easy to follow. 

 An anastomosis takes place between them and this apparently occurs 

 throughout the length of the animal. The posterior nerves are quite 



