330 



abysso-pelagic and pelagic districts also occurs, as well as 

 between the abysso-pelagic and abyssal. 



The marine abyssal districts comprise the lightless depl lis, 

 or generally those depths exceeding one hundred fathoms. 

 As assimilating plant life is absent in these disl ricts,the food 

 supply of ihe organisms existing in them must be wholly 

 derived from the districts in which such assimilating vegeta- 

 tion exists. A large proportion of the food of the abyssal 

 animals is contained in the organic oozes and sediments 

 which constantly settle down in a more or less decomposed 

 state from the lighted districts. The abysso-pelagic district 

 is frequently invaded by organisms from the pelagic district, 

 which descend into the dark regions during the day. 



Bionomic Characteristics of Marine Oroaxis.us. 



Having now considered the life-districts, we will next turn 

 to a study of the organisms inhabiting them. A bionomic 

 classification of animals and plants cannot strictly agree 

 with a classification based on purely anatomical characteris- 

 tics. It is a more primitive classification; but from the 

 point of view here adopted, it is the most convenient. The 

 following five groups, based on the habits and mode of loco- 

 motion of the organisms, will be found to be a convenient, 

 and, from the bionomic point of view, a natural classifica- 

 tion : Plankton, Nekton, Benthos, Mero-pl aide ton, and 

 Pseudo-plankton. 



The term plankton was first introduced by Victor Hensen 

 in 1887. It was derived from the Greek TrAayxTos — mean- 

 ing to drift about aimlessly. In the marine realm it com- 

 prises those organisms which spend their lives in the sea, 

 drifting about from place to place, without sufficient power 

 to direct theirown course.* It comprises organisms ranging 

 from microscopic dimensions to medusa' .">() cm. in diameter. 

 (Walther, '97, p. 211.) While some of the larger animals of 



I follow Walther rather than Ortmann in the definition of the term plankton. The 

 latti-r employs it in a sense which I consider synonymous with pelagic as here defined. The 

 distinction between Plankton and Nekton seems an important one, even though inter- 

 mediate types are common. 



