Adaptations to Environment 121 



The "floating sea-meadows," as Sir John Murray called 

 them, are always receiving contributions from inshore waters, 

 where the conditions are favourable for the prolific multiplica' 

 tion of unicellular Alga?, and there is also a certain amount of 

 non-living sea-dust always being swept out from the seaweed 

 id sea-grass area. 



Swimmers and Drifters 



The animals of the open sea are conveniently divided into 

 the active swimmers (Nekton) and the more passive drifters 

 (Plankton). The swimmers include whales great and small, 

 such birds as the storm petrel, the fish-eating turtles and sea- 

 lakes, such fishes as mackerel and herring, the winged snails or 

 sea-butterflies on which whalebone whales largely feed, some of 

 the active cuttles or squids, various open-sea prawns and their 

 relatives, some worms like the transparent arrow-worm, and such 

 active Protozoa as Noctiluca, whose luminescence makes the 

 waves sparkle in the short summer darkness. Very striking as 

 an instance of the insurgence of life are the sea-skimmers 

 (Halobatidas), wingless insects related to the water-measurers 

 in the ditch. They are found hundreds of miles from land, skim- 

 ming on the surface of the open sea, and diving in stormy weather. 

 They feed on floating dead animals. 



The drifters or easygoing swimmers for there is no hard 

 and fast line are represented, for instance, by the flinty-shelled 

 Radiolarians and certain of the chalk-forming animals (Globi- 

 gerinid Foraminifera) ; by jellyfishes, swimming-bells, and 

 Portuguese men-of-war; by the comb-bearers or Ctenophores; 

 by legions of minute Crustaceans; by strange animals called 

 Salps, related to the sedentary sea-squirts; and by some sluggish 

 fishes like globe-fishes, which often float idly on the surface. 



Open-sea animals tend to be delicately built, with a specific 

 gravity near that of the sea-water, with adaptations, such as pro- 

 jecting filaments, which help flotation, and with capacities of 



