THE PROSOMATIC SEGMENTS OF AMMOCCETES 315 



Upon reaching the opercular and chilarial neuromeres an extra- 

 ordinary exception is found ; the cardiac nerves of these two neuro- 

 meres are fused together, run dorsally, and then form a single nerve 

 called the pericardial nerve, which runs outside the pericardium 

 along the whole length of the mesosomatic region, and gives off a 

 branch to each of the cardiac nerves of the branchial neuromeres as 

 it passes them. 



This observation of Patten and Bedenbaugh shows that the peri- 

 cardial nerve of Limulus agrees with the very nerve postulated by 

 the theory, as far as concerns its origin from the chilarial and 

 opercular neuromeres, its remarkable course along the whole 

 branchial region, and its segmental branches to each branchial 

 segment. 



At present the comparison goes no further ; there is no evidence 

 available to show what is the destination of these segmental branches 

 of the pericardial nerve, and so far all evidence of their having any 

 connection with the veno-pericardial muscles is wanting. Carlson, 

 at my request, endeavoured in the living Limulus to see whether 

 stimulation of the pericardial nerve caused contraction of the veno- 

 pericardial muscles, but was unable to find any such effect. On the 

 contrary, his experimental work indicated that each veno-pericardial 

 muscle received its motor supply from the corresponding mesosomatic 

 ganglion. This is not absolutely conclusive, for if, as Blanchard 

 asserts in the case of the scorpion, a close connection exists between 

 the action of these muscles and of the heart, it is highly probable 

 that their innervation conforms to that of the heart. Now Carlson 

 has shown that this cardiac nerve from the opercular and chilarial 

 neuromeres is an inhibitory nerve to the heart, while -the segmental 

 cardiac nerves belonging to the branchial ganglia are the augmentor 

 nerves of the heart. 



His experiments, then, show that the motor nerves of the heart 

 and of the veno-pericardial muscles run together in the same nerves, 

 but he says nothing of the inhibitory nerves to the latter muscles. 

 If they exist and if they are in accordance with those to the heart, 

 then they ought to run in the pericardial nerve, and would naturally 

 reach the veno-pericardial muscles by the segmental branches of the 

 pericardial nerve. 



Moreover, inhibitory nerves are, in certain cases, curiously 

 associated with sensory fibres ; so that the nerve which corresponds 



