330 



RESPIRATION. 



changing the blood in the lungs, and bringing 

 successive portions of it into contact with the 

 atmospheric air. On the other hand, in some 

 of the most simple forms of animal life, which, 

 with the exception of some of the entozoa, 

 are all aquatic, the function of respiration is 

 effected by the external surface, and they have 

 no special organ for exposing their nutritious 

 juices to the action of the atmospheric air, 

 no apparatus for bringing fresh supplies of 

 the surrounding fluid into contact with their 

 bodies, and no canals or tubes for securing a 

 more rapid change of those portions of the 

 nutritious juices exposed to the action of the 

 atmospheric air. 



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 atmospheric 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 laminae 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- 

 chiae, 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 

 Gecarcimans, though the respiratory organs 

 have a close resemblance to the branchiae 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 onrans for circulating the nutritious fluid 



O ~ 



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 membrane, is the most essential, 

 and the other two may be considered as 

 merely accessory to it. In those animals, as 

 the Infusoria, the Polypes, &c., that have no 

 special organs of respiration, the surfaces, 

 bathed by the fluids in which they live, act as 

 a respiratory membrane: 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 Medusae 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 (Asterida>),and in the sea-urchins (Ec/ii- 



