346 Ihiircrsih/ of Califoniiii l'iibli<(ili<ivs in Zoolofjij. [Vol.4 



Introduction. 



In view of OnIwmIcI's (.-03) abl<' discussion of thi- physical 

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

 suKR'estive presentation of the phenomenon of vertical eirculation 

 in relation to tlie jn-oblems of plankton produetion. jiddi-d interest 

 attaches to all structural modifications and adaptations of the 

 organisms eonstitutinj^' the plankton. New meanint;^ and sifinifi- 

 cance appear in characters which have lonji escaped notice or have 

 been regarded as only additional instances of that wonderful 

 variety of form and wide lan^e of variation within the species 

 which pelafiic organisms are wont to exhibit. 



In the present paper certain .structural features in the skeletal 

 parts of (\ raiium givinj; evidence of changes of functional sitc- 

 niticance 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 Ci nitiinii is noteworthy among pelagic unicellular 

 organisms for the length and tenuity of the proces.ses or horns 

 which extend from the central raidbody containing the nucleus 

 and the main mass of the cytoplasm. In common with the re.st 

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

 exoskeleton. Thesi' 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 seoies of species known in the genus. These 

 horns provide, as I have elsewhei-e ( :(),S) shown, a functional 

 adaptation to flotation and to orientation upon sinking, of prime 

 importance to the species. In the coin-se 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 reueneration 

 in the normal conditions attending the life of the various siu'cies 

 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- 



