Animal Life and Intelligence. 



aquatic animals in a tank or aquarium in which there is 

 no means of supplying fresh oxygen, either by the action 

 of green vegetation, or by a jet of water carrying down air- 

 bubbles, or in some other way. And then there are a 

 number of animals which have no special organs set apart 

 for breathing. In them respiration is carried on by the 

 general surface of the body. The common earthworm is 



mo. 



Fig. 2. — Gills of mussel. 



o.g., outer gill ; i.g., inner gill; mo., mouth; m., muscles for closing shell ; ma., mantle; 

 s., shell; f„ foot; h., position of heart; e.s., exhalent siphon, whence the water passes out 

 from the gill-chamber ; i.s., inhalent fiphon, where "he water enters. 



The left valve of the shell has been removed, and the mantle cut away along the dark line. 



one' of these; and most microscopic organisms are in the 

 same condition. Still, even if there be no special organs 

 for breathing, the process of respiration must be carried on 

 by all animals. 



3. They eat and drink. The living substance of an 

 animal's body is consumed during the progress of those 

 chemical changes which are consequent upon respiration ; 

 and this substance must, therefore, be made good by 

 taking in the materials out of which fresh life-stuff can 

 be formed. This process is called, in popular language, 

 feeding. But the food taken in. is not identical with the 

 life-stuff formed. It has to undergo a number of chemical 

 changes before it can be built into the substance of the 

 organism. In us, and in all the higher animals, there is 

 a complex system of organs set aside for the preparation, 

 digestion, and absorption of the food. But there are 

 certain lowly organisms which can take in food at any 

 portion of their surface, and digest it in any part of their 



