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



the various species of Ceratium represented. The collection had 

 a semi-tropical facies, including such species as C. trichoceros, C. 

 gallicmn and C. biceps. We find in this exuviation an apparent 

 adaptation to changed conditions of flotation. This plankton of 

 semi-tropical aspect may be brought to the San Diego region by 

 occasional invasions of water from the south, possibly by north- 

 ward extensions of the in-shore counter current which has been 

 reported off Lower California.- There is also the possibility that 

 plankton of the warmer surface waters may at times, if not con- 

 stantly, sink to lower and colder levels. Organisms of the plank- 

 ton drifting northward or sinking to lower levels are thus brought 

 into regions of lower temperature with increased molecular fric- 

 tion which makes possible a reduction in specific surface. Such 

 a reduction is brought about by the exuviation of the old theca 

 with its lists and rugose surface and its replacement by the 

 smoother new wall, and also by the dropping off of the outer ends 

 of the antapical horns or of all three horns. 



Normal Autotomy in Ceratium. 



Any observer of marine plankton will have his attention often 

 called to the large number of individuals of Ceratium, especially 

 of the longer-horned forms such as C. biceps, and many species 

 of the C. tripos and C. macroceros groups, in which the horns 

 appear to have been broken oft'. These mutilations are in some 

 cases plainly of the nature of accidental breakages such as might 

 come to pass in the exigencies of life in surface waters or result 

 from collisions and rough handling which these delicate organ- 

 isms undergo in the course of collection in the plankton net and 

 in subsequent treatment of the material. The frequency with 

 which mutilated individuals were found and especially their 

 abundance in certain collections led me to suspect that other 

 agencies than mere accident were at work in causing this phe- 

 nomena in most of the mutilated individuals. A careful exam- 

 ination of accumulated data on this point indicates that autotomy 

 of the horns is a normal phenomenon in Ceratium. The reasons 

 for this conclusion are as follows : 



'^ See Quarterly Current Charts of the Hydrographic Department, British 

 Admiralty ('97). 



