868 SPECIAL PHYSIOLOGY. 



which, beset with fine setae, which retain vesicles of air, is made to reach the 

 surface of the water, so that the creature can breathe whilst the rest of the 

 body is submerged, and the head turned downwards, in order to watch for its 

 prey. The larvae of the Ephemerae, still more curiously, breathe by external 

 tuft-like or leaf-like thoracic or abdominal gills, connected with the tracheae, or 

 by similar organs situated in the intestinal canal ; but in the perfect state, 

 like other Insects, they breathe by tracheae. Aquatic beetles come to the sur- 

 fa.ce to breathe, or carry globules of air below the water. The same habit 

 prevails among certain Arachnida, some of the water-spiders even building 

 nests beneath the water, and carrying down air to them, which they can af- 

 terwards respire. 



Aquatic Respiration. 



In this form of breathing, the physical process concerned is simply that of 

 the moist or false diffusion of gases in a state of solution, those of the blood or 

 nutritive fluids of the animal interchanging with those of the water, which, 

 whether fresh, brackish, or marine, is the universal medium of aquatic respir- 

 ation. That water contains air, is shown by placing it under the air-pump, 

 or by boiling it, which processes abstract or expel the air, and render it unfit 

 to support aquatic animal life. The quantity of free oxygen contained in wa- 

 ter is, of course, much less, volume for volume, than it is in air, inasmuch as 

 the source of the oxygen is the air itself, held in solution. But the air, dis- 

 solved in water, is somewhat richer in oxygen than the ordinary atmosphere ; 

 this is explained by the fact that oxygen is twice as soluble as nitrogen, in 

 that fluid. Nevertheless, the gaseous interchanges in aquatic respiration, are 

 necessarily slower and less energetic than in aerial breathing. The tempera- 

 ture of aquatic breathers is low ; they maintain a heat very little raised above 

 that of the surrounding medium, which is also a much better conductor of 

 heat than air, and so robs them quickly of their caloric ; their movements and 

 other vital acts are comparatively sluggish and slight ; their life is more vege- 

 tative, and even those which have a perfect circulation, are characterized by 

 possessing cold blood. The Mammalian aquatic species, such as seals, por- 

 poises, and whales, though chiefly or entirely inhabiting the water, are yet 

 air-breathing animals provided with lungs, and warm-blooded. 



In water-breathing animals in which a distinct circulation of blood exists, 

 a special respiratory organ is always present, connected with the circulatory 

 apparatus. With the exception of Insects, this is true likewise of air-breath- 

 ing animals. The more perfect the circulation, the more carefully must res- 

 piration be provided for ; otherwise, carbonic acid, accumulating in the blood, 

 would be speedily conveyed to the nervous centres, and produce rapid poison- 

 ing. Such an event is more imminent in the warm-blooded air-breathing, 

 than in the cold-blooded water-breathing animals. 



The most perfect breathing apparatus for aquatic respiration, consists of 

 exceedingly vascular projecting membranous processes of various forms and 

 complexity. In the lower animals, such processes are more simple, and in 

 the absence of a distinct circulation, they are ciliated upon their surface. 

 These projections are named branchia?, or gills; they differ in structure, as 

 much as the lungs of the air-breathing animals. Again, in certain forms of 

 aquatic animals, and in some Entozoa, there are found, with or without gill- 

 like projecting organs, remarkable internal ramified tubes or vessels, commu- 

 nicating, by an opening, with the surface of the body, and partially ciliated in 

 their interior ; these are the so-called water-vessels or water-vascular system ; 

 they appear to have a respiratory office. In still lower aquatic animals, des- 

 titute altogether of vessels, certain hollow ciliated portions of the surface, or 

 interior, of the body, exist, named ciliated sacs; in others, ciliated discs are 

 present ; often the cilia are methodically arranged on the surface, or in the 

 interior of the body ; lastly, in the lowest forms, these microscopic moving 

 organs cover a portion or every part of the minute organism, and so are aux- 

 iliary to respiration. 



Vertebrata. The highest of this Class which have gills, are the Amphibia. 

 In the larval or tadpole condition, all these animals have minute gills, and 





