THE PROSOMATIC SEGMENTS OF AMMOCGETES 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 Redenbaugh 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 Carls6n 
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 
