64 LIFE IN THE SEA [CH. 



between the tide marks. On the other hand plank- 

 tonic organisms like medusae or ctenophores must 

 drift about at the surface, or just beneath the 

 latter in clear water. If they are stranded by tides 

 or gales a thing that often happens they perish. 

 In the course of their evolution of certain types of 

 structure and habit most marine organisms have 

 surrendered the capacity of existing except under 

 very restricted conditions. 



Yet this limitation of habitat is partially com- 

 pensated for by the evolution of larval stages in the 

 life-history of an organism. We nearly always find 

 that a sessile benthic animal has evolved a free- 

 swimming larval stage : or the primitive pelagic form 

 has evolved a sessile habit during the latter period 

 of its life-history. Whichever view we take it is the 

 case that the great majority of animals which live 

 attached to the sea-bottom, or burrowing in the sand 

 or mud, or crawling or swimming along the sea- 

 bottom, begin life as eggs which drift about in the sea, 

 or which may be attached to the parent during their 

 period of incubation. In either case the creature 

 which hatches out from the egg-shell is a larva, and 

 it lives pelagically among the plankton drifting over 

 relatively wide sea areas. This is the meaning of 

 a larval stage in the life-history of an animal 

 it is an interruption of the course of the develop- 

 ment to afford the incompletely formed embryo the 



