490 DISTRIBUTION OF LIFE IN THE OCEAN 



ism to keep afloat without the excessive use of locomotor 

 organs. 



Animals of the Sea Floor. — Our knowledge of the 

 animals living in shallow water is fairly complete, but 

 of life at depths of over a mile much remains to be 

 learned. Such deep-sea animals as are brought to the 

 surface are usually attached to the bottom or are of slug- 

 gish habits, and are thus the ones that are readily cap- 

 tured by nets dragged along the bottom. If one were 

 to drag a net from a balloon along the surface of the 

 earth it is obvious that birds, many insects and other 

 active creatures would rarely if ever be captured. Nev- 

 ertheless a sufficient number of specimens have been 

 secured to enable us to say that every one of the larger 

 groups of animals has its deep-sea representatives. 

 Sponges, corals, worms of widely different appearance, 

 snails, clams, crabs, shrimps, sea urchins, starfishes as 

 well as fishes are present in abundance. Many of these 

 are quite similar to species along shore. Others depart 

 more or less widely in structure from their shallow water 

 relatives. It has also been pointed out that deep-sea 

 animals have a large amount of water in their tissues 

 which enables them to withstand the enormous pressure 

 to which they are subjected. As no green plants live in 

 deep water, the organisms that rain down from the lesser 

 depths serve as food for many deep-sea species that in 

 turn may be preyed i^pon by the others. 



Phosphorescence. — From time immemorial man has 

 known of the phosphorescence of the sea, and for a 

 comparatively long period it has been recognized that 

 it is produced by numerous species of organisms. Never- 

 theless it is only in very recent times that various deep- 

 sea expeditions have given us a fairly accurate notion 

 of its general features. We now know that many uni- 

 cellular animals and plants possess this peculiarity of 

 emitting flashes like those of the firefly, and that some 

 of the fishes, a considerable number of free-swimming 



