FOOD OF DEEP-SEA ANIMALS. 157 



But diatoms contribute indirectly to the nourishment 

 even of the great North Atlantic whales, for these feed 

 chiefly on the acorn-like* medusae, brought to them by the 

 Gulf Stream, which sometimes cover the ocean so thickly 

 as to make it look like a prairie strewn with yellow leaves. 

 Each medusa consists of from five to nine lobes, and as 

 each lobe has been found filled with diatoms to the number 

 of 700,000, each individual must swallow from three and a 

 half to more than six millions at a meal. 



Diatoms have another important function, which will be 

 referred to later, but the countless millions which are for 

 ever dying and sinking must form a considerable item in 

 the bill of fare of many of the creatures which live at the 

 bottom of the sea, and are therefore wholly dependent upon 

 what is brought to them. For there are no beds of weed to 

 which they can go for a meal. Seaweeds require a certain 

 amount of light, and usually form only a fringe about a 

 mile wide round the coast, being practically limited to depths 

 of less than a hundred fathoms, though stragglers are met 

 with here and there. * 



The average depth of the ocean between 60 N. and 

 60 S. is 2,500 fathoms, and the population would be 

 scanty indeed if they lived only on such pieces of weed as 

 are torn from the coast. Besides the deep-sea dwellers, 

 there are not only the Foraminifera already mentioned, but, 

 in the hotter seas, vast multitudes of the wing-footed 

 mollusks, called pteropoda from the fin-like lobes projecting 

 from their sides, which live in and on the surface of 



* The most highly sensitive photograph plates remain unaltered 166 

 feet below the surface. 



