CHAPTER IV. 



LIFE IN THE SEA {continued). 



_:^ We find therefore that there are two main groups of organisms 

 in the plankton of the sea. There are first of all those which are 

 pelagic throughout their lives : these are the permanent plankton. 

 Then we have very many animals which appear only for a short 

 time in the plankton and then forsake this mode of life and 

 adopt either a benthic or a nektic habitat. To the permanent 

 plankton belong the diatoms and the other drifting algae, almost 

 all the protozoa, many of the jelly-fishes, many worms, some 

 molluscs and several very important groups of Crustacea. On the 

 other hand there are also very many organisms which form what we 

 may term the transitory plankton : all these are eggs, larvae or 

 other developing stages of marine organisms, and among them we 

 have the eggs and larvae of fishes, molluscs, Crustacea, echino- 

 derms, worms, with various stages in the development of most 

 zoophytes and actinians, and also the spores of those algae which 

 are in their adult state fixed organisms. For a certain time in 

 the year all these appear as drifting pelagic organisms and then 

 they settle down on the sea bottom, or adopt a definitive nektic 

 mode of life. 



This interchange between the plankton, nekton and benthos 

 depends on the fact that an animal or plant, which for the greater 

 part of its life is rooted to one spot, must possess some means 

 whereby its eggs may be distributed over a much wider area than 

 is occupied by the adult forms. Thus on the land the seeds of 

 plants are carried about by winds and other means, so we find also 

 in the sea that all benthic and many nektic animals produce 



J. F. 6 



