I3O A. J. CARLSON. 



system than any other gasteropod. There are no definite gang- 

 lia, the nerve cells being scattered all along the main nerves or 

 " nerve-cords." So far as I am aware, nerves to be cardiac 

 apparatus of these animals have not been described. Haller 

 (1883) describes nerve cells and nerve fibers in the walls of the 

 auricles, but he does not make out the connection of these nerve 

 fibers with any nerve or nerve-cord outside the heart. My own 

 work was done on Cryptochiton stelleri. This species was selected 

 because of its large size, individuals being frequently found that 

 measured 30 cm. in length. From the lateral and ventral side 

 of the cesophageal nerve ring (PI. IV., Fig. 6) proceed two pairs of 

 nerve-cords. The median pair passes posteriorly into the foot, 

 the lateral pair runs posteriorly in the mantle in a sinus between 

 and slightly dorsal to the efferent and afferent gill sinuses, to 

 unite at the posterior end of the mantle. These are the pleuro- 

 visceral cords. From these cords numereus nerves are given 

 off, some taking a lateral and ventral direction to the gills, others 

 passing dorsally into the mantle and median into the wall of the 

 body cavity. The nerves vary in size, but they are all very small 

 and branch soon after leaving the cord, so that it is difficult to 

 follow them for any great length. In the region of the heart one 

 and sometimes two of these tiny nerves can be followed in the 

 wall of the efferent gill sinuses and on to the auricles (Fig. 6, /). 

 On their course these nerves branch like the rest so that only a 

 small division of the nerve reaches the auricle and this branch 

 cannot be followed in the auricular musculature for any consider- 

 able distance. These are, however, probably not the only nerves 

 that reach the auricles from the pleuro-visceral cords. The 

 auricles are closely attached to the body wall along the side or 

 base, Haller (1883) even claiming that the auricular musculature 

 shades imperceptibly into that of the body wall. Posterior to the 

 opening of the efferent gill sinuses into the auricles, several 

 nerves can be followed to within less than I mm. of the place of 

 their attachment to the body wall (Fig. 6, 2]. It seems there- 

 fore probable that the auricles are supplied, not with one but 

 with several nerve filaments from the nerve cords. No nerves 

 could be traced to the heart or the pericardium from the pedal 

 cords. 



