2195. There are therefore as many vascular organs as there are possible combinations; thus with the tegument, the lung, the intestine, the sexual parts, and with the systems of animal life.
a. CUTANEO-VASCULAR ORGANS—BRANCHIÆ.
2196. The development of the vessels unto a special organ in the integument are Respiratory organs; or, more properly speaking, the development of the integument to constitute a special organ in combination with the vessels, is a respiratory organ.
2197. In the commencement the branchiæ are only a vascular network upon, and therefore subordinate to, the integument. They pass, however, through all possible stages of development, until they have subjected and converted also the integument into a vascular system; a point which is attained in the formation of the lungs.
2198. The branchial membrane already commences in the earthworm to concentrate itself and dilate, so as to form what have been called the sacculi or pouches; in the leech it is folded inwards, so as to form lateral vesicles or cysts, that prognosticate the air-passages or stigmata of insects; in the Nereids the vessels project upon the back from above the integument as free branchial ramules, a formation, which is again found in the nudibranchiate Gasteropods.
2199. These branchial ramules form at first two rows extending along the whole dorsal region; by degrees, however, the posterior set disappear, and the cervical branchiæ only are left, as antetypes of the gills of fishes.
2200. In the Mollusca the branchial vessels unite to form laminæ upon the ventral sides, and are already surrounded by a kind of thoracic cavity, the mantle, as is exemplified in the tectibranchiate Gasteropods. Here again the branchiæ are either a simple vascular net enveloped in a fold of the mantle, or elongated into filaments disposed like the teeth of a comb, or into laminæ, &c.
2201. The lateral branchiæ of the Nereids usually emit filaments resembling feet, upon the root of which the true branchiæ are then placed. In the Crustacea these filaments harden into actual feet provided with joints. The feet are therefore nothing else than branchial filaments, which have lost their vegetative function.
2202. In many worms the same branchial filaments are converted into hairs or bristles, which are therefore none other than desiccated branchial filaments.
2203. Even the hairs of Mammalia and the feathers of Birds have been left as remnants of the original branchial formation.