in] THE FACTORS OF DISTRIBUTION 63 



inhabit some pelagic animal which is drifted for great 

 distances. 



The barriers to the universal distribution of marine 

 organisms are to be found in the different types of 

 structure evolved by them, and in corresponding 

 specialisation in the modes of metabolism acquired 

 during the course of their evolution. Most organisms 

 have developed types of bodily conformation which 

 necessarily restrict them to definite areas of sea or 

 sea-bottom. A shore alga lives attached to a stone 

 on the beach and cannot exist floating permanently 

 at the surface of the sea ; on the other hand a pelagic 

 alga or diatom lives drifting at the sea-surface and 

 cannot always inhabit the sand or mud of the shore. 

 A pelagic fish may roam over extensive tracts of sea- 

 surface, but if it attempts to descend towards the 

 bottom the increasing pressure will be fatal to it; 

 and conversely an abyssal fish might migrate over 

 the entire ocean bottom of the world sea, but it cannot 

 approach the surface without great distension of its 

 swim-bladder, protrusion of its stomach through its 

 mouth, and disintegration of the muscles and other 

 tissues such injuries as would be fatal to it. Sessile 

 benthic animals like zoophytes, barnacles, or sea- 

 anemones must live on the beach or at the sea-bottom 

 attached to stones, and cannot drift about at the 

 surface; while littoral animals such as the cockle 

 or lugworm must restrict themselves to the shore 



