22 LIFE IN THE SEA [CH. 



sea water, no matter how clear it may be, contains 

 abundance of life in the form of organisms too small 

 to be seen without the aid of the microscope. We 

 can always capture these organisms by filtering even 

 a tumblerful of water through a sieve made of the 

 very fine silk cloth that millers use in separating the 

 various grades of flour. It would be rare indeed for 

 as much sea water as could be lifted in an ordinary 

 thimble to be without a dozen or more microscopic 

 organisms, and at the time of maximum abundance 

 of life in the sea such a quantity of water might 

 contain several thousands of living creatures. All 

 these organisms which can so be obtained are grouped 

 together to form our third great category of marine 

 life the Plankton. The general character of all is 

 that they are unable to make migrations which can 

 materially influence their distribution. Many of 

 them do not possess any organs of locomotion, and 

 even if they do these organs are quite ineffective for 

 that purpose. Planktonic animals and plants are 

 simply drifted about passively in the sea by the 

 I agencies of tidal streams, currents, and winds. 



The composition of the plankton is remarkably 

 \ variable. Looking at it from a general point of view 

 \we can recognise in its constitution two great classes 

 fof organisms. (1) Larval stages of all the great 

 groups of life : thus fishes which live either demersally 

 or pelagically spawn eggs which, as a rule, drift about 



