Influence of Nutrients on Animals 293 



grass, for example, contains spicules of silica so hard that only rumi- 

 nant animals with specially adapted teeth can feed on it as a regular 

 diet. 



The necessity for alacrity and for the possession of suitable ana- 

 tomical adaptation for dealing mechanically with the prey is suffi- 

 ciently evident, but not so immediately apparent is the necessity for 

 the animal to possess the digestive equipment required to deal chem- 

 ically with the food once it is inside. Carnivores do not live on hay 

 —nor do ruminants live on meat. In studying the food sources of 

 animals the possible lack of enzymes for digesting materials that ap- 

 pear to be abundant and available in the environment is a diflBculty 

 sometimes overlooked, especially in relation to lower animals. For a 

 long time it was supposed that clams, oysters, and other common 

 bivalves obtained their nourishment chieHy from the larger, more 

 conspicuous species of diatoms filtered from the plankton. Most of 

 these larger types of phytoplankton are now known to pass through the 

 intestines of the shellfish quite undigested. Physiological investiga- 

 tion has revealed the fact that these mollusks have no extracellular 

 cellulase or protease. Hence, they are quite incapable of digesting 

 relatively large food particles that are enclosed in an intact cellulose 

 or protein cell wall ( Coe, 1948 ) . The nutritional needs of filter feed- 

 ers like clams and mussels appear to be satisfied by the ingestion of 

 organic detritus or of a type of nannoplankton small enough to be en- 

 gulfed by digestive phagocytes. 



Many specializations have arisen in relation to the mechanical and 

 chemical requirements of food getting in natural habitats ( AUee et al., 

 1949, Ch. 17). Although most animals are clearly either herbivorous 

 or carnivorous, these categories are not absolute and some species 

 like the opossum are omnivorous in their diet. Some carnivores eat 

 considerable plant material. Herbivores occasionally consume 

 animal food, as is exemplified by the fact that reindeer have been 

 known to eat fish when no other food was available. The diet of 

 the black bear ranges all the way from sizable mammals to tiny ants, 

 and from blueberries and honey to salmon. 



Many species of animals are highly modified for feeding on special 

 parts of plants. The aphids and scale insects have sucking mouth- 

 parts enabling them to obtain the sap of their hosts. Some insects 

 are adapted for boring into and digesting, the wood, the bark, or the 

 cambium of trees. Not only do certain insects feed exclusively on 

 the leaves of plants, as do many mammals and a few reptiles, but in 

 addition the "leaf miners," which live between the leaf surfaces, are 

 further specialized to feed exclusively on the softer interior tissue. 



