. The depths of the ocean : a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. "Michael Sars" North Atlantic Deep-sea Expedition (1910); Oceanography; North Atlantic Ocean. 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.
. The depths of the ocean : a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. "Michael Sars" North Atlantic Deep-sea Expedition (1910); Oceanography; North Atlantic Ocean. 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 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.). Pleas
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