146 THE STUDY OF ANIMAL LIFE CHAP. 



warm-blooded, and the same is true of hibernating 

 mammals. In the winter-sleep the attempt to keep up 

 the warm-blooded regime is abandoned. 



In the same connection it should be noted that there 

 is an electrical change associated with every muscular 

 contraction, and that this production of electricity (from 

 modified muscle) becomes important in the Gymnotus, 

 the torpedo, and other electric fishes. On another line, 

 and very much more frequent, is the production of light, 

 sometimes from within the substance of the cells of the 

 luminescent organs and sometimes from a secretion ex- 

 uded from these cells. 



4. Nutritive Functions. The energy expended in ex- 

 ternal and internal work is made good again by the 

 raw materials taken into the body proteids, carbo- 

 hydrates, fats, water, and some salts. Whereas green 

 plants can feed at a very low chemical level, getting 

 carbon supplies from CO 2 and nitrogen supplies from 

 nitrates and the like, animals must have carbohydrates, 

 fats, and proteids already worked up by plants or by 

 other animals. 



The first step in nutrition is ingestion, which presents 

 no problems of much theoretical difficulty. The Amoeba 

 engulfs its neighbour, the sponge wafts in minute organ- 

 isms and particles, the starfish protrudes its stomach 

 on its victim, the sea-cucumber thrusts its muddy ten- 

 tacles into its mouth, the medicinal leech sucks in blood, 

 the oyster w r afts in diatoms and the like by means of 

 cilia on its gills, the shark takes big bites, the frog shoots 

 out its tongue on the insect, the python gets laboriously 

 outside its big booty, the humming-bird sips nectar 

 with its long tongue, the baleen whale catches myriads 

 of minute molluscs in the sieve of whalebone which 

 hangs down from the roof of its great buccal cavity. A 

 radical distinction of importance is between the tough- 

 mouthed animals that have something in the way of 

 hard parts about the mouth and the tender-mouthed 

 which feed mainly on small organisms and debris. Or 

 we may classify animals as (1) carnivorous, (2) vege- 

 tarian, (3) those of mixed diet to whose mill many things 



