20 THE PROTOZOA THE DAWN OF LIFE 



set free the oxygen; while by absorbing the nitrogen and other 

 elements in the form of mineral salts in solution in the water, it is 

 thus nourished like a typical plant. At the same time the move- 

 ments of the flagellum have been seen to create a current by 

 which minute fragments of organic matter are propelled down the 

 gullet into the soft internal protoplasm, where they are digested, so 

 that a characteristically animal mode of nutrition also takes place. 



Among the Flagellata are to be found the bulk of those marine 

 organisms which form a very large proportion of what is called 

 the micro-plankton of the sea, and which may be said to consti- 

 tute the primary food-supply of the higher forms of marine life. 

 Therefore, investigations which will help to throw light upon 

 their life-histories, and the conditions which are most favourable 

 to their development and multiplication, will be of the greatest 

 value in the consideration of many problems connected with 

 fishery questions, and the successful rearing of marketable marine 

 fishes in the early stages of their lives. 



The marine Dinoflagellata, which chiefly compose this 

 floating plankton, are in many instances phosphorescent, lighting 

 up the sea on a dark night with their wonderful glow so that every 

 wave seems to break in a cascade of silvery light. They are char- 

 acterised by the presence of two flagella : one is conspicuous 

 and filiform, arising in a longitudinal groove extending its whole 

 length and projecting beyond the animal ; the other also arises 

 in the longitudinal groove, but is band-like in appearance and 

 extends along a somewhat spiral transverse groove from which it 

 never protrudes during life, but in its movements resembles a 

 girdle of cilia a deceptive appearance which led to these organisms 

 formerly being called Cilioftagellata. The true character of this 

 second flagellum was discovered by Klebs. Multiplication usually 

 takes place by oblique division of the body into two dissimilar 

 halves, each half undergoing a peculiar growth to reconstruct the 

 missing portion. 



Most of the Dinoflagellata possess a complete membrane or 

 cuirass of cellulose, the typical material of the cell-walls of plants, 

 usually hard, with distinctive form and markings, and divided 

 into plates. Cemtium, which is remarkable for the horn-like 

 backward prolongations of its lower end, is an interesting and 

 abundant genus in the sea. 



