clude the attempt to describe it in every particular case; but the normal or prevailing arrangement is nearly the following:—All the minute internal ramifications converge into several trunks, which gradually become wider till they debut on a spiracle. Tracing the course of these trunks upwards from the spiracle, they are generally found to divide into two principal branches, one of them becoming ramified ad infinitum, the other of a more uniform diameter and running across to unite with a similar one from the opposite side. A more complete union, however, is established among all the principal branches, by two longitudinal trunks extending the whole length of the body, one on each side, not far from the margin and parallel with it. These are nearly of the same diameter throughout, and intersect the transverse branches not far from their origin. This continual anastomosis provides for the utmost freedom of conveyance in every direction, and places the stigmata on opposite sides of the body in direct communication with each other. In addition to these there is sometimes a longitudinal tube in the centre, and in certain instances there are not fewer than five or six running lengthwise. Some of the principal longitudinal tubes give off branches to the legs, and a considerable number enter the cavity of the head.[1] Although the tracheæ
- ↑ To illustrate the distribution of the air-vessels, we have given a figure of those of Nepa cinerea after M. Léon Dufour, See Pl. III. fig. 9, in which a, a, a, represent the lateral spiracles, in this instance obsolete, because the air is admitted by terminal tubes; b, b, ventral spiracles, at the base of the
