D.— ZOOLOGY, 87 



the mode of life of an aina^ba. That anuvha and its allies, tlie 

 Khizopcxls, are descended from a flagellate ancestor is a view suggested 

 by Lankester'^ in 1909, which was adopted by Doflein,'- and is now 

 strongly advocated by Pascher'* as a result of much new research. 



The transformation from the plant to the animal mode of feeding 

 we can see iu action by studying actual organisms which exist to-day. 

 In the course of my work already referred to on the culture of plankton 

 organisms there has on several occasions flourished in the flasks a small 

 flagellate belonging to the group of Chrysomonads, which was first 

 described by W'ysotzky under the name of Pedinella hexacostata, and 

 to which I drew the attention of Section D at the Cardiff Meeting in 

 1920. The general fomi of Pedinella resembles that of the common 

 Vorticella, but its size is much smaller. The body, which is only about 

 5" in diameter, is shaped like tlie bowl of a wine-glass, and from the 

 base of the bciwl, which is the posterior end, a short, stiff stalk extends. 

 From the centre of the anterior surface there arises a single long 

 flagellum, surrounded at a little distance by a circle of short, stiff, proto- 

 plasmic hairs. Arranged in an equatorial ring just inside the body 

 are six or eight brownish-green chromatophores or chloroplasts. In a 

 healthy culture Pedinella swims about freely by means of a spiral move- 

 ment of the flagellum, which functions as a tractor, the stalk trailing 

 behind. The chromatophoi'es are large, brightly coloured and well 

 developed, and the oi-ganism 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 trailing stalk. 

 The immediate effect of this fixing is that a current of water, produced 

 by the still vibrating flagellum, streams towards the anterior surface 

 of the body, and small particles in the water, such as bacteria, become 

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

 ing 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. Tn some of the cultures in which bacteria were very plentiful 

 nearly all the Pedinella remained fixed and fed in the animal way, and 

 when this was so the chromatophores had almost disappeared, though 

 they could still be seen as minute dark dots. ^Ye 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 tlie naked 

 and armoured peridinians belong — the same transition from plant to 

 animal nutrition can be well followed by studying different mem- 

 bers of the group. In heavily armoured forms, with a rich supply of 

 chromatophores, nutrition is chiefly plant-like or holophytic. In those 

 with fewer chromatophores there is, on the other hand, often distinct 

 evidence of the ingestion of other organisms, and nutrition becomes 



11 Lankester. Tieatif^e on Zoolofji/, Part I., London, 1909, p. xxii. 

 '- Doflein, Protozoenkunde, 1916. 



I'' Pascher. Archir f. Protixtenlunde . Bd. 36, 1916. p. 81. and Bd. 38 1917 

 p. 1. 



