ii] RHYTHMICAL CHANGE IN THE SEA 45 



by jumps, immediately after the animal has cast its 

 shell. It has happened that a crab has crept into 

 an empty bottle just after casting, and then having 

 grown to its annual amount during the next week or 

 so it finds itself unable to leave the bottle. In the 

 case of animals growing in this way the growth 

 function is a ' discontinuous ' one. 



Thus some time in the spring months a wave of 

 production of organic substance in the form of new 

 individuals sweeps across the sea, dying away in the 

 summer months. This wave is exhibited in the annual 

 spawning of almost all the higher animals and it 

 affects the composition of the plankton. Just as 

 certainly as the vernal equinox approaches and the 

 sea temperature rises, so does the mass of planktonic 

 animals and plants inhabiting a unit volume of sea- 

 water increase to its annual maximum, and then 

 decrease again. Now we must clearly distinguish 

 between the increase in the mass of the plankton due 

 to the myriads of eggs and larvae spawned by the 

 fishes and larger invertebrates, and that due to the 

 reproduction of the pelagic diatoms, algae, protozoa, 

 and micro-crustacea. The former have only a tran- 

 sitory planktonic phase in their life-histories, the 

 latter persist in the plankton throughout their entire 

 lives. 



The transitory plankton includes a great number 

 of species each of which is strictly periodic in its 



