456 BIOLOGY AND HUMAN LIFE 



considered a great delicacy. The parts of fishes remaining after 

 the food portions and oil have been removed are sometimes 

 ground into a meal for feeding animals, and all wastes are useful 

 as fertilizer. In many ponds the fishes help to keep down the mos- 

 quito nuisance by devouring the eggs and larvae of the insects. 



On the other side of the account, several species of fish are un- 

 desirable inhabitants of fresh waters, the pike and the pickerel 

 being destructive of more valuable forms ; and along the coasts 

 many varieties of sharks are not only destructive to smaller 

 fish, crabs, lobsters, etc., but sometimes attack men. 



With growing populations and narrowing margins of usable 

 soil more and more attention has to be given to the utilization 

 of food supplies from the waters of the earth. The United 

 States Bureau of Fisheries is constantly investigating problems 

 connected with increasing fish supplies, protecting food fishes, 

 introducing new forms, restocking lakes and rivers, and so on. 



337. Batrachians. The name amphibians, which is sometimes 

 given to this class, suggests that they have two modes of life, 

 one in the water and one in the air or on land. This refers to 

 the breathing, which is always by means of gills in the young 

 stage and (in the more developed forms) by means of lungs in 

 the adult stage. The name amphibian does not apply to all of 

 them, for there are some that continue to breathe by gills all 

 through life. The metamorphosis characteristic of the develop- 

 ment in the frogs and salamanders (see section 227 and Fig. 138) 

 is absent in a few forms, which continue in what we should call 

 the tadpole stage throughout life ; but among some of these it 

 is possible to force a further development by reducing the 

 moisture. 



There are three main groups of batrachians : ( i ) those that 

 have no legs and so resemble snakes without scales; (2) those 

 that have tails as well as legs — newts, salamanders, etc. ; and 

 (3) those that have legs but no tails in the adult stage — frogs 

 and toads. None of the living forms grow to be very large, the 

 giant salamander of Japan being perhaps the largest, attaining 

 a length of from four to five feet ; it is used as food. In ancient 



