128 A. J. CARLSON. 



6). From the dorsal side of the visceral ganglion two nerves are 

 given off similar to those described in Mytilus. One of these 

 nerves passes to the posterior adductor muscle and the dorsal body 

 wall of the anal region ; the other nerve (Fig. 2, 3) also sends 

 branches to the adductor muscle but the main branch passes 

 dorsally to the body wall where it takes an anterior direction and 

 can be followed into the dorsal wall of the pericardial cavity. 

 On its course branches are given off to the kidney, the aorta, 

 and the rectum (Fig. j, 3 and 4). This nerve is evidently 

 homologous with the dorsal mantle nerve in Mytilns (Fig. I, 4). 

 In Tapes staminca the visceral ganglion is situated in the angle 

 made by the posterior adductor muscle and the adductor muscles 

 of the foot. The ganglion gives rise to five pairs of nerves, viz., 

 the posterior mantle nerves (i), the nerves to the siphon (2), the 

 great osphradio-branchial nerves (4), the cerebro-visceral commis- 

 sures (cvconi), and a pair of tiny nerves to the posterior adductor 

 muscle (3). In the figure these last nerves are turned to the 

 side so as to be represented in the same plane, as they turn in a 

 dorsal direction almost at right angles to the other nerves. The 

 commissures and the branchial nerves run parallel and close 

 together for a distance of about one centimeter from the ganglion, 

 and then the branchial nerves turn laterally and anteriorly into 

 the osphradium and the gills. At the level of the pericardial 

 cavity the commissures penetrate the reproductive gland, to 

 which they send fibers. Close to the visceral ganglion each com- 

 missure gives off a small nerve (5) which runs parallel with it for a 

 short distance and then turns laterally to enter the kidney and 

 the heart. These nerves may be designated the reno-cardiac 

 nerves, and are evidently homologous with the nerves marked 4 

 in Fig. 2. The renal organs occupy the space between the adduc- 

 tor muscles of the foot and the pericardial cavity and extend along 

 the base of the gills. The nerves branch so extensively in the 

 kidney that I was not able to follow any one branch directly on 

 to the auricles, but the physiological evidence is conclusive that 

 these nerves send branches to the heart. The nerves to the 

 ventricle enter that organ solely through the auricular walls, as 

 severence of the anterior and the posterior aortas together with 

 the rectum at either end of the ventricle does not interfere with 



