THE ANIMAL AND ITS ENVIRONMENT. 



103 



OUTGO OF LUMBRICUS. 



Of the daily outgo the water, carbon dioxide, and salts are 



/ o 



devoid of energy, but the urea contains a small amount which is 

 a sheer loss to the animal. Were the earthworm a perfect ma- 

 chine it could use this residue of energy by decomposing the urea 

 into simpler compounds [viz., ammonia (jSTH 3 ), carbon dioxide 

 (CO 2 ), and water (H 2 O)] ; but it lacks this power, though there 

 are certain organisms (Bacteria) which are able to utilize the last 

 traces of energy in urea (p. 107). To the daily outgo must be 

 added the occasional loss both of matter and of energy suffered 

 in giving rise to ova and spermatozoa, and in providing a certain 

 amount of food and protection for the next generation. 



Interaction of the Animal and the Environment. The action 

 of the environment upon the animal has already been sufficiently 

 stated (p. 97). It remains to point out the changes worked by 

 the animal on the environment. These changes are of two 

 kinds, mechanical (or physical) and chemical. The most impor- 

 tant of the former is the continual transformation of the soil 

 which the worms effect, as Darwin showed, by bringing the 

 deeper layers to the surface, where they are exposed to the at- 

 mosphere, and also by dragging superficial objects into the bur- 

 rows. The chemical changes are still more significant. The 



