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 to 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 tiuid 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 m 
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 
