METABOLISM AND TRANSPORT IN ANIMALS 



513 



its sticky tongue. Many fish depend on speed 

 for overhauling their prey. Birds that feed 

 on animals likewise rely on the speed of 

 flight as well as on sharp claws and hooked 

 beaks. Claws and teeth are familiar mam- 

 malian weapons of offense. 



Food selection 



The fact that we can classify animals ac- 

 cording to their diet indicates that they se- 

 lect their food. As already noted, amoebas 

 seem to prefer the small flagellate Chilo- 

 monas. Hydras feed largely on such small 

 aquatic animals as water fleas, worms, and 

 insect larvae. The mouth parts of insects are 

 supplied with sense organs that probably aid 

 them in distinguishing different kinds of 

 food. Many plant-eating insects and their 

 larvae feed on one kind of plant, or on a 

 small group of plants, and will starve if these 

 are not available. Vertebrates likewise are 

 more or less strictly limited in the character 

 of the food they eat. This has led to such 

 common names as fish hawk for the osprey, 

 carrion crow for the black vulture, duck 

 hawk, herring gull, kingfisher, flycatcher, 

 anteater, sea cow, and fruit bat. 



The character of the food depends 

 largely on the habitat of the animal, whether 

 marine, fresh-water, or terrestrial; on its 

 period of activity, whether diurnal or noc- 

 turnal; on the size of the eater; on its 

 method and speed of locomotion; on the 

 structure of its organs or organelles for cap- 

 turing its prey; on the type of its mouth 

 parts; and on the structure of the digestive 

 system. Sense organs also play an important 

 role in food selection. These factors are all 

 important and account in general for the 

 character of an animal's food; but we still do 

 not know why in many cases one type of 

 food is preferred to some other type that 

 seems to us to be equally desirable. 



Ingestion of food 



In many animals, the structures used in 

 obtaining food are also employed for pur- 



poses of ingestion. This is true of the 

 pseudopodia of amoebas, the cilia of para- 

 mecia, the tentacles of hydras, the muscular 

 pharynx of planarians, earthworms, and cer- 

 tain sucking insects, the cilia of clams, and 

 the tongue of the frog. In other animals, 

 special methods of ingestion are employed. 

 In sponges, food particles drawn into the 

 central cavity by means of flagella are en- 

 gulfed by the collar cells. In hydras, the 

 mouth and body wall force the food into 

 the gastrovascular cavity, but small particles 

 are ingested by the nutritive cells of the 

 inner body wall. In the crayfish and in 

 many insects, the food is held by certain of 

 the mouth parts while it is crushed or bitten 

 into small pieces by the mandibles or jaws. 

 A starfish does not ingest the large bivalves 

 it attacks, but everts its stomach, inserts it 

 between the valves, and digests the soft 

 parts of the prey outside its body. The lam- 

 prey attaches to fishes with its suckerlike 

 mouth and horny teeth, and rasps away the 

 flesh with its tongue, then feeds on blood. 

 Most fish do not chew their food but hold 

 it with their teeth and swallow it at once. 

 Many other vertebrates, such as snakes and 

 birds, swallow their food whole. Mammals, 

 for the most part, chew their food before 

 swallowing it; their teeth are modified for 

 this purpose; for example, the grinding teeth 

 of herbivorous types (cattle, horses, etc.), 

 and the cutting teeth of carnivores (dogs, 

 cats, etc. ) . The relation between the struc- 

 ture of the teeth and the character of the 

 food in mammals is worth detailed study. 



DIGESTION AND 

 DIGESTIVE SYSTEMS 



Digestion 



The apparatus between the mouth and 

 the anus is for getting food into the blood. 

 The process of digestion is the breaking 

 down of food material so that it is in a 

 soluble form and can pass through mem- 

 branes and protoplasm; that is, it can be 



