Page:Proceedings of the Royal Society of London Vol 4.djvu/486

Rh fleeted movements of parts both anterior and posterior to an irritated limb ; as that of the commissural set does the movements of parts situated on the opposite side of the body to that which is irritated. In the ganglia of the cord in lulus and Polydesmus, the fibres of the inferior longitudinal series are enlarged and softened on entering the ganglion, but are again reduced to their original size on leaving it ; thus appearing to illustrate the structure of ganglia in general. In the developement of the ganglia and nei^ves in these genera, and also in Geophilus, the same changes take place as those which were formerly described by the author as occurring in insects ; namely, an aggregation of ganglia in certain portions of the cord, and shift- ing of the position of certain nerves, which at first exist at ganglionic portions of the cord, but afterwards become removed to a non- ganglionic portion. The nervous cord is elongated, in order that it may keep pace with the growth of the body, which is periodically acquiring additional segments : that this elongation takes place in the ganglia is proved by these changes of position in the nerves lying transversely across the ganglia. The author infers from these facts, that the ganglia are centres of growth and nourishment, as well as of reflex movements, and that they are analogous to the enlarge- ments of the cord in the vertebrata.

A series of experiments on the Iidus and Lithobius are next re- lated; the result of which shows that the two supra-oesophageal gan- glia are exclusively the centres of volition, and may therefore strictly be regarded as performing the functions of a brain : so that when these ganglia are injured or removed, all the movements of the ani- mal are of a reflex character. When, on the other hand, these gan- glia are uninjured, the animal movements are voluntary, and there exists sensibility to pam : there is, however, no positive evidence that the power of sensation does not also reside in the other ganglia.

The second part of the paper relates to the organs of circulation. In all the ?^Iyriapoda and Arachnida the dorsal vessel or heart is di- vided, as in insects, into several compartments, in number coiTe- sponding to the abdominal segments. Its anterior portion is divided, immediately behind the basilar segment of the head, into three di- stinct trunks. The middle portion, which is the continuation of the vessel itself, passes forwards along the cesophagus, and is distributed to the head itself; while the two others, passing laterally outwards and downwards in an arched direction, form a vascular collar round the oesophagus, beneath which they unite in a single vessel, as was first noticed by IMr. Lord in the Scolopendra. This single median vessel lies above the abdominal nervous cord, and is extended back- wards throughout the whole length of the body as far as the termi- nal ganglia of the cord, under which it is subdivided into separate branches accompanying the terminal nerves to their final distribu- tion. Immediately anterior to each ganglion of the cord, this vessel gives off^^ a pair of vascular trunks ; and each of these trunks is di- vided into four arterial vessels, one of which is given to each of the principal nerves proceeding from the ganglion, and may be tmced along with it to a considerable distance. Of these, the vessel situated