PHYSIOLOGY OF THE INVERTEBRATE HEART. 1 2/ 



anterior course enters a small ganglion situated dorsal to the hind 

 gut at the junction of the body wall with the pericardium (10). 

 The two ganglia are connected by a tiny nerve. Several small 

 nerves are given off anteriorly from the ganglia to the pericardum 

 and the gut, but I was unable to trace any of these branches on 

 to the musculature of the ventricle. There are individual varia- 

 tions as regards the place of branching of the nerves and the two 

 sides are usually not symmetrical. If the ganglia on the course 

 of these nerves correspond to the "cardiac ganglia" of Quatre- 

 fages, there is this difference to be noted that in Mytihis the 

 ganglia are not situated in the pericardial cavity and that branches 

 of the nerves can be traced to the base of the auricles but not on 

 to the ventricle. The two nerves in Mytihis are probably more 

 of dorsal mantle nerves than of cardiac nerves, for even where 

 the nerves ramify in the pericardium, the pericardium and the 

 body wall are fused into one. In most of the gasteropods the 

 renal and the cardiac nerves are united in one common trunk. 

 In Mytihis the renal nerves are given off from the cerebro-visceral 

 commissures (i i) and they can be followed throughout the whole 

 length of the kidney at the base of the gill, but I was unable to 

 trace any of the branches on to the auricles or the ventricle. 



The visceral nervous complex of Mya (PI. IV., Figs. 2 and 3) 

 differs somewhat from that of Mytilus. The visceral ganglia are 

 fused into one with enlargements at the point of origin of the 

 branchial nerves, and there are in most cases slight ganglionic 

 enlargements at the cerebro-visceral commissures a little anterior 

 to their union with the visceral ganglion. One or two small 

 nerves pass laterally from each commissure to the ganglionic 

 protuberance at the point of origin of the branchial nerves (Fig. 

 2, 3). From the commissures close to the visceral ganglion two 

 small nerves take their origin (Fig. 2, 4). After an anastomosing 

 with the nerves from the commissures to the sides of the visceral 

 ganglion these nerves take a postero-lateral course on the surface 

 of the kidney between the commissures and the branchial nerves. 

 Branches of these nerves can be traced into the substance 

 of the kidney and to the base of the auricle. Posterior to the 

 point of origin of the branchial nerves another pair of tiny 

 nerves are given off to the lateral surface of the kidney (Fig. 2, 



