316 THE ORIGIN OF VERTEBRATES 
to the pericardial nerve, viz. the branchialis profundus of the facial, 
may be an inhibitory and sensory nerve, and not motor at all. Miss 
Alcock’s observations are purely histological; no physiological 
experiments have been made. 
At present, then, it does not seem to me possible to say that 
Carlson’s experiments have disproved any connection of the peri- 
cardial nerve with the veno-pericardial muscles. We do not know 
what is the destination of its segmental branches; they may still 
supply the veno-pericardial muscles even if they do not cause them 
to contract; they certainly do not appear to pass directly into them, 
for they pass into the segmental cardiac nerves, and can only reach 
the muscles in conjunction with their motor nerves. Such a course 
would not be improbable when it is borne in mind how, in the frog, 
the augmentor nerves run with the inhibitory along the whole length 
of the vagus nerve. 
Until further evidence is given both as to the function of the seg- 
mental branches of the pericardial nerve in the Limulus, and of the 
branchialis profundus in Ammoccetes, it is impossible, I think, to 
consider that the phylogenetic origin of these tubular muscles is as 
firmly established as is that of most of the other organs already 
considered. I must say, my own bias is strongly in favour of looking 
upon them as the last trace of the veno-pericardial system of muscles, 
a view which is distinctly strengthened by Carlson’s statement that 
the latter system contracts synchronously with the respiratory move- 
ments, for undoubtedly in Ammoccetes their function is entirely 
respiratory. Then again, although at present there is no evidence to 
connect the pericardial nerve in Limulus with this veno-pericardial 
system of muscles, yet it is extraordinarily significant that in such 
animals as Limulus and Ammoccetes, in both of which the mesoso- 
matic or respiratory region is so markedly segmental, an intrusive 
nerve should, in each case, extend through the whole region, giving 
off branches to each segment. Still more striking is it that this 
nerve should arise from the foremost mesosomatic and the last pro- 
somatic neuromeres in Limulus—the opercular and chilarial segments 
—-precisely the same neuromeres which give origin to the correspond- 
ing nerve in Ammoccetes, for according to my theory of the origin of 
vertebrates, the nerves which supplied the opercular and metastomal 
appendages have become the facial nerve and the lower lip-branch 
of the trigeminal nerve. 
