34 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



issues a long flagellum originating by a double root from the lining of 

 the reservoir. In the centre or slightly nearer the pointed end is a large 

 rounded nucleus, and close to the reservoir is a contractile vacuole (c.v) 



which expels its water into the reser- 

 voir. The animal is bright green in 

 colour, the chlorophyll being situated in 

 rounded discs — the chromatophores or 

 chloroplasts {chr) — which are arranged 

 in a single layer just below the surface. 

 The chlorophyll is not the only colour- 

 ing matter. Close to the contractile 

 vacuole and just beneath the surface is a 

 shining orange-red spot — the stigma (5) 

 — which is physiologically an eye — not 

 an eye which can " see " in the ordinary 

 sense but merely a portion of the proto- 

 plasm sensitive to light and enabling the 

 creature to detect from which direction 

 the light is coming. 



As regards the life-history one of the 

 chief peculiarities is a negative feature 

 — the absence so far as is known of 

 any process of syngamy. Ordinarily 

 reproduction takes place by fission, the 

 Euglena splitting longitudinally from 

 the flagellar end. As a rule the Euglena 

 is encysted in a clear jelly during the . 

 process and the young individuals 

 surround themselves also with a cyst 

 and then divide again. In this way 

 great masses of jelly are formed with 

 the dividing Euglenas contained in it. 

 These, buoyed up by bubbles of gas, 

 often form a conspicuous green scum 

 on the surface of pools. 



As regards the physiology of 

 Euglena features of special interest are 

 presented by its movements and by its mode of nutrition. The Euglena 

 swims by means of its flagellum which is directed forwards and describes 

 a conical or spiral kind of movement in such a way that the body of the 

 Euglena is as it were towed after it. Such a type of flagellum is some- 



chn' 



N. 



Fig. 14. 



Euglenj. c.v. Contractile vacuole; chr, 

 chroraatophore ; ft, flagellum; N, nucleus ; 

 p, paramylum ; r, reservoir ; s, stigma. 



