Numerous and interesting modifications of the respiratory apparatus, each wonderfully adapted to the wants of the individual animal, and the medium in which it lives, and in ad mirable relation to its other nutritive func tions, fill up the wide interval between the most complex and the simplest methods of carrying on the function of respiration. This, like the other functions of the body, is, in proportion to the energy of its manifestations, more concentrated upon individual organs chiefly or entirely constructed for this pur pose, and it thus becomes more and more specialized as we ascend in the zoological scale.
The position of the respiratory apparatus is chiefly regulated by the circumstance of the animal being terrestrial or aquatic, or, in other words, by its supply of atmospheric air being in the gasiform condition, or held in solution by water. In the greater number of aquatic animals, the respiratory apparatus is placed on or near the external surface of the body ; while, in the terrestrial animals, it is situated more or less deeply in the interior of the body. The medium in which the animal lives also influences the size and complexity of its respiratory organs. As the quantity of atinospheric air in contact with respiratory organs of the same extent of sur face must be much smaller in aquatic than in terrestrial animals, a more extended respira tory organ is required in the former than in the latter to effect the same amount of re spiration, just as a more extended digestive apparatus is required in herbivorous than in carnivorous animals to extract the same amount of nutritious matters from their food. As water cannot furnish to terrestrial animals an adequate supply of atmospheric air, their vital actions are brought to a stand when their respiratory organs are immersed in that fluid, and this the more quickly in those im mediately dependant, as the birds and mam malia, upon large and frequent supplies of atmospheric air. The respiratory organs of aquatic animals are, on the other hand, in adequate for the performance of respiration in the atmospheric air, but from very different circumstances. The most obvious of these are, 1st, their respiratory organs, from their external position, are either freely exposed or only partially covered, so that when they are removed from the water into the atmosphere they become dry in consequence of the eva poration of their moisture, and this the more rapidly as there is little or no provision in dependent of the water in which they live, for keeping these surfaces moist by a secretion, as in air-breathing animals : 2dly, in those cases where the respiratory organ consists of nu merous membranous plates or laminm that float apart in the water, and have every por tion of their external surface bathed in this fluid, removal into the atmospheric air causes them to fall together, so that comparatively a small quantity of their surface is now in contact with the air. In some of the Crusta
cea, and in some fishes, as the eels, the bran chim, or respiratory organs, being covered to a great extent, desiccation proceeds slowly, and life may be prolonged for a considerable time in the atmospheric air. In one of the groups of the Crustaceans, the land-crabs or Gecarcinians, though the respiratory organs have a close resemblance to the branchim of the aquatic tribes, yet as they inhabit damp situations, and have a provision for keeping the respiratory surface moist, they are enabled to live as terrestrial animals.
It has already been stated that the respira tory apparatus in all the higher animals con sists of three distinct parts ; —of an expanded membrane, through which the atmospheric air and the nutritious fluid the blood, act chemically on each other ; of organs for re newing the atmospheric air in contact with the external surface of this membrane ; and of organs for circulating the nutritious fluid along channels placed upon the inner surface of this membrane. Of these three portions of the respiratory apparatus, the first, or the ex panded membrane, which may be termed the respiratory menzbrane, is the most essential, and the other two may be considered as merely accessory to it. In those animals, as the Infusoria, the Polypes, •Scc., that have no special organs of respiration, the surfaces, bathed by the fluids in which they live, act as a respiratory menzbrane : the atmospheric air in the surrounding fluids is there brought into contact with the nutritious juices, and the function of respiration is effected in the feeble manner in which it is manifested in such animals. In those animals possessing special organs of respiration, the respiratory mem brane is formed in almost all cases by pro longations, folds, or reduplications of the internal or external tegumentary membrane, and all of these different arrangements are evidently with the view of increasing the extent of surface of that membrane. In the pulmograde Medusm the margin of the disk, though smooth, and presenting no prolonga tion of the external tegumentary membrane, acts more efficiently in the function of respira tion than the other parts of the surfaces of the body, and may be considered as a re spiratory organ, in consequence of a large quantity of the nutritious juices flowing through numerous vessels distributed there. In some of the Echinodermata, as in the star fishes (Astericke), and in the sea-urchins (Echi niche), the internal integumentary membrane is materially aided in the performance of this function by the aquiferous canals in the former, and by the peritoneal cavity in the latter ; indeed, the peritoneal cavity may be considered the special organ of respiration in the Echinidm.