THE PELAGIC FAUNA AND FLORA. 205 



the smooth streaks of the surface formed by the currents and 

 winds. The mollusks in their turn, according to their size and 

 activity, are either eaten or are the eaters of some animals more 

 helpless than themselves. While the pelagic forms can move 

 about from place to place in search of more abundant food, 

 the more sedentary types must depend on the supply dropped 

 in their immediate vicinity, or brought to them by currents. 



It is dif&cult, in this gigantic struggle for food going on 

 among the pelagic animals, to trace the effect of protective agen- 

 cies. While undoubtedly we seem to be able to satisfy ourselves 

 as to the efficiency of various causes, such as transparency, the 

 extraordinary develoj)ment of locomotory organs, or coloration, 

 to take the pelagic types at certain stages out of reach of dan- 

 ger, yet there are in the course of the development of nearly all 

 these pelagic types long periods during which the embryos are 

 more than ever subject to destruction, at the very time when 

 they would seem to have specially adapted themselves to sur- 

 rounding circumstances. Probably at no time during the life of 

 many crustaceans do they run so imminent a risk of destruction 

 as during their zoea stage. (Figs. 129, 130, 131.) Left at the 

 mercy of every current or ripple, they fall a prey to acalephs, to 

 polyps, to cephalopods, and especially to young fishes, in spite of 

 the huge appendages which seem at first sight to serve as pro- 

 tests against being swallowed by their smaller enemies. It 

 would carry me too far to repeat in detail how many of our 

 fishes devour minute organisms ; how many mollusks and polyps 

 live upon the most diminutive denizens of the seas, both animal 

 and vegetable ; how the echinoderms find their food by swallow- 

 ing mud filled with living foraminif era or dead organic matter ; 

 and how the minute pelagic Crustacea act as scavengers upon all 

 dead animals. 



The quantity of food contained even in apparently clear sea- 

 water can readily be tested by leaving a shallow dish of it to 

 settle during the night, and examining the bottom on the fol- 

 lowing day, when it will be found to be covered by a consider- 

 able amount of fine silt, made up of animal and vegetable 

 fragments. (Fig. 132.) 



It can readily be seen that, as far as the deep-sea forms are 



