PLANKTON 261 



assemblages captured in cold polar waters are of different 

 ages and stages, young and adults of several generations 

 occurring together in profusion, ^ and it is supposed that the 

 adults " may be ten, twenty or more years of age." At the 

 low temperature the action of putrefactive bacteria and of 

 enzymes is very slow or in abeyance, and the vital actions of 

 the Crustacea take place more slowly and the individual 

 lives are longer. On the other hand, in the warmer waters 

 of the tropics the action of the bacteria is more rapid, 

 metabolism in general is more active, and the various stages 

 in the life -history are passed through more rapidly, so 

 that the smaller organisms of equatorial seas probably only 

 live for days or weeks in place of years. 



This explanation, if confirmed, may account also for the 

 much greater quantity of benthonic organisms which has 

 been found so often on the sea-floor in polar waters. It is 

 a curious fact that the development of the polar marine 

 animals is in general ' direct ' without larval pelagic stages, 

 the result being that the young settle down on the floor of 

 the ocean in the neighbourhood of the parent forms, so 

 that there come to be enormous congregations of the same 

 kind of animal within a limited area, and the dredge will in 

 a particular haul come up filled with hundreds, it may be, 

 of an Echinoderm, a Sponge, a Crustacean, a Brachiopod, 

 or an Ascidian ; whereas in warmer seas the young pass 

 through a pelagic stage and so become more widely dis- 

 tributed over the fioor of the ocean. The " Challenger " 

 expedition found in the Antarctic certain Echinoderms, for 

 example, which had young in various stages of development 

 attached to some part of the body of the parents, whereas 

 in temperate or tropical regions the same class of animals 

 set free their eggs and the development proceeds in the open 

 water quite independently of, and it may be far distant from, 

 the parent animal. 



1 Whether, however, the low temperature may not also retard 

 reproduction is worthy of consideration. 



