NUTRITION 



call breathing or respiration — the taking in of pxygen 

 and giving out of carbonic-acid gas — does not require 

 special organs in the lower animals. In these it is ac- 

 complished by epithelial cells, which clothe the surface 

 of the body — the ectoderm of the outer skin layer and 

 the entoderm of the inner gut-covering. As nearly all 

 these coelenteria live in the water, or (as parasites) in 

 some fluid that contains air, and as these fluids are con- 

 stantly pouring in and out of the body, the exchange of 

 gases is accomplished at the same time. But in the 

 higher animals this is rarely found, only in the small 

 animals of simple construction (such as the rotifers and 

 other vermalia, and the smallest specimens of the mol- 

 lusca and articulata). The majority of these coelomaria 

 attain a considerable size, and so require special organs; 

 these afford a larger surface for the exchange of gases in 

 the limited space, and accomplish a very peculiar chemi- 

 cal work as the localized organs of respiration. They 

 fall into two groups according to the nature of the en- 

 vironment; gills for breathing in water and lungs for 

 breathing on land. The latter take the oxygen directly 

 from the atmosphere, and the former from the water, 

 in which atmosphere air is contained in solution. 



The instruments of water-respiration which we call 

 gills (branchice) are generally attenuated parts or proc- 

 esses of the outer skin or the inner gastric skin ; hence 

 we distinguish the two chief forms, external and inter- 

 nal gills. Both are richly provided with blood-vessels 

 which bring the blood from the body for the purpose 

 of aeration. Cutaneous or external gills are especially 

 found in the vertebrates, in the form of threads, combs, 

 leaves, pencils, tufts of feathers, etc., which are drawn 

 out from the entoderm as local processes of the outer 

 skin, and afford a wide surface for the interchange of 

 gases between the body and the water. In the moUusca 

 there are usually a pair of comb-shaped gills near the 



229 



