3 16 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. 



