PELAGIC PLANT LIFE 



351 



sweep round to the front, sometimes in a direction parallel to 

 the frontal horn, and sometimes with a moderate amount of 

 divergence. We have already mentioned that C. ardicum and 

 C. longipes belong to the Tricho-plankton and that C. inter- 

 medium and C. macrocej^os are Styli-plankton. We have finally 

 a whole series of variations belonging to the tropical Desmo- 

 plankton, namely C. vultur, C. paviliardii, C. trickoceros, and 

 C. tenue, which we reproduce from Jorgensen's excellent mono- 

 graph (see Fig. ^^ ^, 

 247), and many 

 others. They 

 illustrate the dif- 

 ferent tendencies 

 to variation. In 

 similar fashion 

 there are series of 

 variations which 

 group themselves 

 round the other 

 main types of the 

 genus. 



Guiding forms 

 like these are of 

 very great assist- 

 ance in defining 

 the boundaries of 

 adjacent currents 

 which have a 

 different biological 

 character. But 

 we need to exer- 

 cise the utmost 

 care in drawing 

 conclusions as to 

 the origin of ocean-currents from the composition of their 

 pelagic flora, and it must not by any means be taken for 

 granted that areas where the same species occur are neces- 

 sarily united by a continuous stream connection. We have 

 repeatedly made discoveries which go to indicate that most 

 plankton-species of any consequence are to be found scattered 

 about here and there outside their proper domain, so that 

 these stray individuals might easily originate an abundant 

 flora whenever conditions of existence became favourable. 



Fig. 247.— Species of Ceratium belonging to the type 

 of c. macroceros, tropical species. 



a, C. paviliardii (\*-) ; b, C. trickoceros (-\4) ; c, C. vultur, var. 

 Japonica (\*-) ; d, C. tenue, var. buceros {-\^). (Jorgensen. ) 



