PHYSIOLOGY OF THE INVERTEBRATE HEART. 137 



(4) supplies the gill. These two nerves reach their destination 

 by a circuitous route, passing ventral to the heart. The third nerve 

 (3) is the visceral nerve par excellence, because it sends branches 

 to the liver, the reproductive glands, the heart and the renal 

 organ. Arising from the postero-lateral side of the ganglion this 

 nerve takes a direction towards the aortic sinus and the pericar- 

 dium, but before it reaches these structures it bifurcates. The 

 branch that passes ventral to the aortic sinus (7) gives off a branch 

 to the liver (10) and one or two very small branches to the aortic 

 sinus (12), while the main trunk (11) follows an artery to the 

 reproductive glands. The other nerve (6) takes a postero-lateral 

 direction in the ventral wall of the pericardial cavity. On reach- 

 ing the postero-lateral side of the pericardial cavity it takes a 

 median direction parallel with long axis of the heart. Several 

 small branches are given off by the nerve to the pericardium, and 

 at the level of the auricle a tiny branch (9) is sent forward in the 

 ventral wall of the pericardium and on to the auricle, while the 

 main branch (8) enters the kidney. The nerve on the aortic 

 sinus (8) could not be traced on to the ventricular musculature 

 with certainty, but we shall presently see that nerves do enter the 

 ventricle at the aortic end and that these nerve fibers leave the 

 visceral ganglion in the trunk of the posterior nerve of the left 

 side of the ganglion (3). From the posterior side of the visceral 

 ganglion one or two small nerves (14) are given off which ramify 

 in the musculature and connective tissue of the dorsum immedi- 

 ately adjoining the ganglion and cannot be followed for any dis- 

 tance. From the right pedal ganglion arises a small nerve (13) 

 which connects with the right side of the visceral ganglion. The 

 corresponding nerve of the left side does not come into connec- 

 tion with the visceral ganglion but enters directly into the mus- 

 culature of the dorsum a little anterior to the pericardial cavity. 



Electrical stimulation of either of these two nerves produces 

 none of the effects caused by the stimulation of the pleuro-vis- 

 ceral connectives. 



The visceral nervous system of Bnlla (PI. VI., Fig. 13) differs 

 from that of Aplysia mainly by the presence of five or six ganglia 

 on the course of the nerves. The left commissure has a small ean- 



o 



glion on its course before reaching the main visceral ganglion. 



