452 



THE POPULAR SCIENCE MONTHLY. 



Respiration by the Unmodified Skin and Food-Tract. The 

 general surface of the body has in all soft-skinned animals an aerating 

 or respiratory action ; and the simplest expression of organs is found 

 in those animals where the body-surface is the only breathing-organ. 



But a simpler expression of the function is 

 shown in an amoeba, for example, where by 

 the amoeboid movements every part of the 

 structureless body eventually becomes sur- 

 face, and is brought into immediate contact 

 with the water. The amoeba has no per- 

 manent skin, and no organs of any kind ; 

 consequently it breathes without special or- 

 gans, and the other nutritive functions are 

 equally destitute of them. 



Many animals of higher groups breathe 

 mainly or entirely by the skin. Among ar- 

 Fig. 4 -Tubicola. a, serpuiacon- ticulates, the leech and earth-worm are 



tortuplicata, showing the bran- ' 



chiasand operculum; b, Spirorbis examples. In these a net-work of minute 



communis. ,-,-, i -it i-i-it 



blood-vessels is spread beneath the delicate 

 skin, thus bringing the blood into proximity with the water or air. 

 The lowest crustaceans, and some sea-snails, and sea-spiders also, re- 

 spire by the skin alone. 



Most amphibians use the skin largely in respiration. In cold water, 

 frogs will breathe entirely by the skin, and can not be killed by forced 

 immersion, so long as they are provided with food. Even at ordinary 



Fig. 5. Errant Annelides. A, hairy-hait (NepMkys); B, sea-mouse {Aphrodite)', C, lob-worru 



(Arenicola). (After Uosse.) 



temperature, it is easier to drown some fishes by preventing them from 

 reaching air than it is to suffocate frogs under the same conditions ; 

 yet the former have only gills, and the latter lungs alone. The hell- 

 bender, found in our Western rivers the largest American salaman- 



