CHAPTER VIII 



DISTRIBUTIOX, LOCAL DISTRIBUTION, HISTORICAL 



DISCUSSION 



The dinoflagellates are widely distributed in both fresh and marine waters, 

 from the polar seas to the tropics, reaching their maximum development, numer- 

 ically as well as in speciation and in structm-e, in the warm temperate and 

 tro2)ical regions. No formulation of the laws which undoubtedly govern their 

 distrilnition can he attempted at the present time, owing to the utter lack of 

 anything like adequate records of the occurrence of this group, that is, adequate 

 from a systematic, distributional or seasonal standpoint. Any conclusions that 

 may be drawn are therefore wholly pro^-isional. 



The thecate dinoflagellates have been more fully investigated than have the 

 naked forms, but the records here also are incomplete. It is probable that the 

 two groups are very closely bound together in the temperature and other re- 

 lations which condition their geograpliical distribution, but further investi- 

 gation will be required for the C^innodinioidae at least before these relations 

 can be stated with any degree of certainty. 



Many of the earlier records of the occurrence of these forms, in both fresh 

 and marine waters, are of doubtful value, owing to the often contradictory and 

 poorly defined species characters which usually result in some confusion and 

 error when records of occurrences of species in a large quantity of plankton 

 are made. In the case of doubtful species, where these records are unaccom- 

 panied by figures of the organisms found, the records are sometimes practically 

 useless. 



In plankton forms which have no fixed spatial relations to the substrate the 

 division into littoral, neritic and pelagic life zones is much harder to maintain 

 than is the case with those organisms whose connection with the substrate is 

 even temporary or Avhose swimming habits afford some resistance to a passive 

 drifting with the current. Neither of these factors operates to maintain an 

 establislied position for the majority of the protozoan plankton, hence we find 

 the Protozoa more widely distrilnited through these three zones of oceanic life 

 than are many other groups. This is true of the dinoflagellates to a very large 

 extent, yet we find even here a slight tendency towards segregation of more or 

 less distinct faunal groups. 



The Dinoflagellata are tyjDical pelagic organisms, with some cosmopolitan 

 species and many others of seemingly more limited distribution. They have 

 representatives which are restricted solely to the high seas or are, at most, found 

 only as stragglers in coastal waters, as, for example, Ceratocorys, Histioneis, 



[94] 



