THE VITAL FUNCTIONS OF ANIMALS. 21 



The tube in which this first process of nutrition is effected, is a 

 continuation of the skin. In some very simple animals, where the 

 whole body is composed of a homogeneous mass (ex. gr. in Polyps) 

 there is properly no special intestinal canal. The body is simply 

 excavated, and the internal surface has the same structure as the 

 external. Such creatures may be turned inside out, like the finger 

 of a glove, without dying in consequence : nutrition can proceed 

 undisturbed. Such animals are entirely intestinal canal, independ- 

 ently vital stomachs. The external skin also corresponds in func- 

 tion with the surface of the canal. The skin has the function of 

 Imbibition, which may be compared with absorption by the intesti- 

 nal tube : and on the entire internal surface of the intestinal canal 

 there is evaporation, which corresponds to that of the skin, and 

 with the diminution of this increases. 



In some very simple kinds of animal there is in the intestinal 

 canal only a single opening, which allows the food fo enter and 

 the refuse to escape. In the rest the two openings are separate. 



The Chyle, or nutrient juice which has been produced by 

 digestion, is in many animals immediately poured into the forma- 

 tive tissue of the entire body, and so serves for the nutrition of the 

 different parts. In others it is mixed with a nutrient fluid of higher 

 rank, the blood, which circulates in a system of vessels ; this 

 motion is called Circulation. The vessels which carry the blood 

 towards the parts are called Arteries : those which carry back the 

 blood from the parts towards the center of the circulation are called 

 Veins. This motion is ordinarily assisted and regulated by one or 

 more muscular organs, called Heart. But the chyle is not sufficient 

 to renew the venous blood and render it fit for the nutrition of the 

 parts. It must be brought in contact with atmospheric air, and so 

 be submitted to change before passing into the arterial stream. 

 This function is called Respiration, and the mechanism for it is in 

 different creatures so variously contrived, that it is often difficult to 

 harmonise such variety with the poverty of our language, accus- 

 tomed to include every form under Gills and Lungs. In the case 

 of Lungs, the medium that serves for respiration, mostly air, pene- 

 trates the cavities whose external surface is bathed with blood. In 

 the case of Gills, the medium, here mostly water, does not pene- 

 trate within the tissue, but only bathes the surface on which the 

 blood-vessels are spread out. Gills have very different forms, as of 



