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Show 202 MB. A. D. MICHAEL ON AN [Mar. 5, existence might be anticipated; it will be seen by fig. 20 that I find more than one branch to each leg of the two hind pairs, in the fourth leg in particular I find several branches. Croneberg, Schaub, and Nalepa all found that each of the four leg-nerves on each side of the body was accompanied by a much smaller nerve running parallel to it, which they call the accessory nerve; they all describe and figure it, doubtless correctly, as springing from the brain itself. In the present species the arrangement is very different; in the first two pairs of legs these accessory nerves exist (fig. 20, na.), and may be plainly seen in dissections although they are small and fine'; but they do not spring directly from the brain as in the cases observed by those authors': they spring from the respective principal leg-nerves a short distance from the brain, and are in fact the first branches of those nerves; indeed the only ones which I have traced, although probably others exist in the more distal parts of the principal nerve. The two hind pairs of legs are entirely without accessory nerves, either springing from the brain or from the principal nerves. It is true that branches a good deal like the accessory nerves in character spring from the principal nerve much further on its course (fig. 20, n 3, n 4), but they are so very much further away from the brain than the branches of the nerves of the two first pairs of legs, that they can hardly be considered the homologues of the accessory nerves; moreover these branches are paired, not azygous as the accessory nerves are. No one has traced the accessory nerves to their destination or offered any explanation of what they are. It seems to m e that the present species probably affords the key to this problem ; they are apparently really branches of the principal nerves, which, in the species described by Croneberg and others, and probably in the majority of allied species, have for some reason gradually come to spring more and more closely to the brain until at last they have ended by springing from the brain itself and not from the principal nerve at all. The last pair of large nerves springing from the lower ganglion are a pair quite at the rear and near the median line (fig. 20, ng.), which innervate the genital organs; practically all authors are agreed upon this point. In the present species I find that the principal trunk of the nerve runs to the dorsal side of the genital apparatus, and there gives off numerous fine branches to the various parts ; and also sends a large branch to the vagina or ductus ejaculatorius and penial canal, as the case may be, and the muscles which surround it; and this branch divides, sending secondary branches to the so-called genital suckers. The principal branch forms a distinct ganglion, from which the fine nerves that are distributed to the organs actually arise ; and there are at least one or two small ganglia in connection with the larger trunk. The existence of such ganglia has been alreadv indicated by Schaub and even by Pagenstecher in I860. The branches from this |
