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 difficult, 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 development 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. lt 
would earry 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 foraminifera 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 
