2247. The perfect organ of respiration is an air-organ or lung; combined with the tegumentary system, it is a water-organ or branchia.
2248. There are dermo-branchiæ or the branchiæ proper, as in the Worms, Molluscs, Snails, and Crabs.
2249. Intestinal branchiæ in the Holothuriæ, constituting in the higher animals a spleen.
2250. The sexual branchiæ are probably the primordial kidneys.
2251. The branchiæ when united with the osseous system are the branchial arches of Fishes, which subsequently separate into larynx and thyroid gland.
2252. The self-substantial development of the integument into the respiratory organ is a lung.
2253. When united with the vascular system or the vegetative systems generally, the integument forms the tracheal system of Insects.
2254. The tracheæ are spiral vessels as in plants.
2255. The respiratory organ, united with the motor system, is the lung proper, which is situated within the thorax, and covered by ribs.
2256. The proper lung divides also, like the intestine, into two parts, into the cystiform, pharyngeal expansion of the larynx, and into the pulmonic substance, which is equivalent to the stomach, and wherein the analysis of the air takes place.