346 University of California Publications in Zoology. [Vol.4 



Introduction. 



In view of Ostwald's ( :03) able discussion of the physical 

 factors of the pelagic environment and Nathanson's ( :06) recent 

 suggestive presentation of the phenomenon of vertical circulation 

 in relation to the problems of plankton production, added interest 

 attaches to all structural modifications and adaptations of the 

 organisms constituting the plankton. New meaning and signifi- 

 cance appear in characters which have long escaped notice or have 

 been regarded as only additional instances of that wonderful 

 variety of form and wide range of variation within the species 

 which pelagic organisms are wont to exhibit. 



In the present paper certain structural features in the skeletal 

 parts of Cei-atimn giving evidence of changes of functional sig- 

 nificance are described and these changes are correlated with the 

 problem of flotation as modified by the vertical distribution of 

 temperatures and the possibilities of vertical circulation of ocean- 

 ic waters. 



The genus Ceratium is noteworthy among pelagic unicellular 

 organisms foi' the length and tenuity of the processes or horns 

 which extend from the central midbody containing the nucleus 

 and the main mass of the cytoplasm. In common with the rest 

 of the body they are sheathed within a porulate cellulose wall or 

 exoskeleton. These processes form the single apical and the two 

 (in a few cases, three or more) antapical horns whose position, 

 length, structure and angles of divergence exhibit a most remark- 

 able variety in the scores of species known in the genus. These 

 horns provide, as I have elsewhere ( :08) shown, a functional 

 adaptation to flotation and to orientation upon sinking, of prime 

 importance to the species. In the course of my examination of 

 the Dinoflagellates of the plankton of the Pacific off San Diego 

 during the past seven years I have noted numerous instances of 

 individuals with broken horns and many cases of regeneration, 

 in the normal conditions attending the life of the various species 

 in the pelagic habitat. It is the purpose of this paper to discuss 

 these phenomena and the related one of exuviation in normal con- 

 ditions of pelagic life, and inquire into their significance, es- 



