atory tubes, which were not properly understood till explained by M. Leon Dufour. Externally they have the appearance of an ovipositor, and have often been mistaken for such, although they exist in both sexes. Each is composed of two pieces, grooved internally, and meeting each other very closely at the edges, thus forming an elongated siphon, which introduces the air to two spiracles placed at the termination of the two principal air-tubes running along each side of the body. In this and analogous instances, the other spiracles have become obsolete, their functions being superseded by another kind of mechanism.
In the cases hitherto alluded to, whether relating to terrestrial or aquatic insects, the pneumatic mechanism is adapted for the admission of air from the atmosphere; it is next to be considered as fitted to separate and imbibe the air which is mechanically mixed with water. The species possessed of such an apparatus are as exclusively aquatic as fishes, and the apparatus itself is quite analogous to the respiratory mechanism of these animals. The Gills or Branchiæ are processes of the epidermis, projecting from the body in various forms, commonly resembling hairs or leaves, and completely filled with delicate tracheæ. The exterior membrane is so fine as to allow these to be seen through it. The manner in which the air is extracted by their means is founded on the following law of organic chemistry, recently discovered by M. Dutrochet. If we enclose a liquid or a gas in a cavity with permeable coats, a bladder for example, and plunge this bladder into another liquid or gas
