494 THE AMERICAN NATURALIST [Vol/ LVI 



ture Pedinella swims about freely by means of a spiral 

 movement of the flagellum, which functions as a tractor, 

 the stalk trailing behind. The chromatophores are large, 

 brightly colored and well developed, and the organism 

 is obviously nourishing itself after the manner of a plant, 

 like any other Chrysomonad. But from time to time a 

 Pedinella will suddenly fix itself by the point of the trail- 

 ing stalk. The immediate effect of this fixing is that a 

 current of water, produced by the still vibrating flagel- 

 lum, streams towards the anterior surface of the body, 

 and small particles in the w^ater, such as bacteria, become 

 caught up on the anterior surface, the ring of fine stiff hairs 

 surrounding the base of the flagellum being doubtless of 

 great assistance in the capture of this food. One can 

 clearly see bacteria and small fragments of similar size 

 engulfed by the protoplasm of the anterior face of the 

 Pedinella and taken into the body. The organism is now 

 feeding as an animal. In some of the cultures in which 

 bacteria were very plentiful nearly all the Pedinella re- 

 mained fixed and fed in the animal w^ay, and w^hen this 

 was so the chromatophores had almost disappeared, 

 though they could still be seen as minute dark dots. We 

 can, as it were, in this one organism see the transition 

 from plant to animal brought about by the simple process 

 of the freely swimming form becoming fixed. 



In the group of Dinoflagellates, also— the group to 

 which the naked and armored peridinians belong— the 

 same transition from plant to animal nutrition can be 

 well followed by studying different members of the group. 

 In heavily armored forms, with a rich supply of chro- 

 matophores, nutrition is chiefly plant-like or holophytic. 

 In those wdth fewer chromatophores there is, on the other 

 hand, often distinct evidence of the ingestion of other 

 organisms, and nutrition becomes partly animal-like. 

 Amongst the naked Dinoflagellates such holozoic nutri- 

 tion is very much developed, and in many species has 

 entirely superseded the earlier method of carbonic acid 

 assimilation. 



It is really surprising how many structural features 



