24 THE SEAS 



or bony fish, with the cartilage converted into bone by 

 the presence of limy matter, and the separate gill openings 

 covered over with a flap known as the operculum. 



All animals higher in the scale of evolution than the fish are 

 air breathers but a number have returned to the sea and 

 spend their lives in it, some always near or on the surface 

 but others capable of diving to considerable depths though 

 always compelled to return to the surface from time to time 

 to obtain air. Among the Reptilia are the water snakes 

 or Hydrophtd^: which have keeled bodies and tails flattened 

 like those of fish to aid them in swimming, and the turtles 

 which are aquatic tortoises with the limbs flattened to 

 form swimming paddles. In the Mammalia there are the 

 many kinds of whales, dolphins and porpoises which con- 

 stitute the Cetacea and spend their entire lives in the sea, 

 an existence for which they are just as well equipped as the 

 fish themselves, and also the strange sea cows (Sirenia), 

 sluggish, harmless beasts which usually live in shallow 

 water near the mouths of rivers in tropical regions and 

 frequently sit on their tails with only their heads out of 

 water — the origin, probably, of all the stories about 

 mermaids ! Finally there is a group of the Carnivores 

 (which include the lions, tigers and bears) the members 

 of which, exclusively marine, except in the breeding 

 season, are known as the Pinnipedia and include the seals, 

 sea lions and walruses. 



Plants 



The sea is far poorer in types of plant life than the earth- 

 The flowering plants have very few marine representatives, 

 the best known being the eelgrass (Zostera marina) which is 

 widespread in sheltered regions round our coasts. The 

 great majority of marine plants are Algae, of simpler 

 structure than the flowering plants and reproducing them- 



