A large air-tube (trachea) lies under the skin on each side of the body of lar vae, and opens externally by nine aper tures (stigmata) : it produces on the in side the same number of trunks of air vessels (branchise.) which are distribut ed over the body in innumerable ramifi cations.
Both the trachea and branchim are of a shining silvery colour ; and their princi pal membrane consists of spiral fibres. The most numerous and minute. ramifi cations are distributed on the alimentiry canal.
There is great variety in the number and situation of the external openings, by which insects receive their air.
In most instances the stigmata are plac ed on both sides of the body. The at mospheric air enters by an opening at the end of the abdomen in several aquatic larva:, and even perfect insects. A very remarkable change in this respect takes place in several animals of this class dur ing their metamorphosis. Thus in the larva of the common knats (culex pi piens,) the air enters by an opening on the abdomen : while in the nympha of the same animal it gains admission by twu apertures on the head.
In the class of vermes, which compre hends such very different animals, the structure of the respiratory organs is pro portionally various. Some orders, as those which inhabit corals, the proper zoophytes, and perhaps the intestinal woems, appear to be entirely destitute of these organs : so that if any vital func tion, analogous to respiration, is carried on in these animals, it must be effected by methods which yet remain to be dis covered.
Those vermes, however, which are furnished with proper organs of respira tion, have the same variety in their struc ture which was remarked in insects.. Some, as the cuttlefish, oyster, &c. have a species of gills, varying in structure in different instances. But the greatest number have air-vessels, or trachew. Several of the testaceous vermes have both kinds of respiratory organs. In some of the inhabitants of bivalve shells, as the genus venus, the air-vessels lie be tween the membranes of a simple or double tubular canal, found at the ante rior part of the animal, and capable of voluntary extension and retraction. It serves also for other purposes, as lay ing the eggs. The margins of its mouth are beset with the openings of the trachea.
In the terrestrial gasteropodous mol luscs, of which we may instance the snail and slug, there is a cavity in the neck receiving air by a small aperture, which can be opened or shut at the will of the animal. The pulnionary vessels ramify on the sides of the cavity.