Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so that the nerve which corresponds {316}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 pericardial 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 segmental branches of the pericardial nerve in the Limulus, and of the _branchialis profundus_ in Ammocoetes, 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 movements, for undoubtedly in Ammocoetes 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 Ammocoetes, in both of which the mesosomatic 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 prosomatic neuromeres in Limulus--the opercular and chilarial segments--precisely the same neuromeres which give origin to the corresponding nerve in Ammocoetes, 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.