:T/LU: 



PHYLUM CHORD AT A 



233 



compressed and swim actively about seeking for food. 

 They are voracious feeders on fish and other animals; there- 

 fore, are readily caught on a hoojUL Though one species 

 attains a length of over thirty feet and sometimes attacks 

 man, most sharks are harmless, except for their ravages 

 among fishes and other small animals. They are of some 



p 



FIG. 92. FIG. 93. 



FIG. 92. Three cross sections (A, B, C) of an embryonic vertebrate to show 

 formation of nervous system, chorda, and coelom. 6, membrane surrounding 

 ccelom; c, chorda; e, enteron; n, nerve tube. 



FIG. 93. Section through a placoid scale of an elasmobranch. d, dentine; 

 p, "pulp;" x, enamel; y, integument. 



value as fertilizer^ and for the oil which is extracted from 

 their livers. The' flesh is also canned to some extent and 

 sold as a cheap substitute for the true fishes. Sharks cause 

 great losses by destroying lobsters and valuable food fishes. 

 , The skates and rays have broad flat bodies and commonly 

 lurk on the bottom of the ocean awaiting small animals 



