RESPIRATORY ORGANS. 353 



animal is small there are often no special organs for its ex- 

 ternal respiration, its general surface being sufficient (espe- 

 cially in aquatic animals with a moist skin) to permit of all 

 the gaseous exchange that is necessary. In the simplest 

 creatures, indeed, there is even no blood, the cell or cells 

 composing them taking up for themselves from their en- 

 vironment the oxygen which they need, and passing out 

 into it their carbon dioxide waste; in other words, there is 

 .no differentiation of the external and internal respirations. 

 When, however, an animal is larger many of its cells are so 

 far from a free surface that they cannot transact this give- 

 .and-take with the surrounding medium directly, and the 

 Mood, or some liquid representing it in this respect, serves 

 as a middleman between the living tissues and the external 

 oxygen; and then one usually finds special respiratory or- 

 gans developed, into which the blood is brought to replace 

 its oxygen loss and get rid of its excess of carbon dioxide. 

 In aquatic animals such organs take commonly the form 

 of gills; these are protrusions of the body over which a 

 constant current of water, containing oxygen in solu- 

 tion, is kept up; and in which blood capillaries form a 

 close network immediately beneath the surface. In air- 

 breathing animals a different arrangement is usually found. 

 In some, as frogs, it is true, the skin is kept moist and 

 serves as an important respiratory organ, large quanti- 

 ties of venous blood being sent to it for aeration. But for 

 the occurrence of the necessary gaseous diffusion, the skin 

 must be kept very moist, and this, in a terrestrial animal, 

 necessitates a great amount of secretion by the cutaneous 

 glands to compensate for evaporation; accordingly in most 

 land animals the air is carried into the body by tubes 

 with narrow external orifices, and so the drying up of 

 "the breathing surfaces is greatly diminished; jiL-t as water 

 in a bottle with a narrow neck will evaporate much more 

 slowly than *ho same amount exposed in an open dish. In 

 insects (as bees, butterflies, and beetles) the air is carried 

 by tubes which split up into extremely fine branches and 

 ramify all through the body, even down to the individual 

 tissue elements, which thus carry on their gaseous exchanges 



