172 THE MASTIGOPHORA 



One of the commonest of the Euglenoidea is Euglena viridis, a 

 species which is frequently found in shallow ditches and puddles, 

 giving the water a green tint or forming a green scum on its- 

 surface. The free -swimming individuals are about O'l mm. in 

 length, provided with a single flagellum arising just in front of a 

 short funnel-shaped cytostome at the pointed anterior end of the 

 body. Opening into the cytostome funnel there is a small reservoir, 

 which itself receives the fluids discharged by a system of minute 

 contractile vacuoles. The chlorophyll is present in the form of 

 numerous minute chloroplasts, and the paramylum in the form of 

 many minute plates. At the base of the flagellum there is a red 

 eye -spot composed of numerous granules of " haematochrome." 

 There is a single nucleus. An important phase in the life-history 

 is the resting stage. The individuals swarm to the surface of the 

 water, where they form the green scum. Each individual in the 

 scum loses its flagellum, and, secreting a gelatinous substance 

 which joins with that of its neighbour's to form a continuous jelly,, 

 encysts. 



Division of the nucleus and cell-substance takes place during 

 the resting stage at night. The mi to tic changes commence about 

 two hours after dark and are completed in five hours. The 

 nucleus has in the resting stage a centrally placed " nucleolo- 

 centrosome." This becomes dumb-bell-shaped and then elongates- 

 in mitosis. The chromosomes become parallel to this body, and 

 eventually form an equatorial ring round it. In this position they 

 undergo longitudinal splitting (Keuten). 



Euglena undergoes several successive divisions under the same 

 cyst-membrane, forming quadrants, octants, etc., but all result from 

 successive longitudinal division, unlike the ciliate infusorian Colpoda,. 

 which produces similar clusters resulting from successive cross- 

 division. Again, in the Volvocines the clusters arise by alternate 

 longitudinal and transverse division (Klebs). 



Thus, in the case of Euglena and Copromonas, division takes- 

 place after the loss or withdrawal of the flagellum, but in the allied 

 Astasiina division takes place during the motile phase. 



Euglena gracilis occurs in both green and colourless conditions, 

 so that, employing Pfeffer's terminology, it may be at one time 

 autotrophic (holophytic), at another time heterotrophic (sapro- 

 phytic), the two conditions being connected by a mixotrophic 

 transition. 



An important contribution to the life-history of the Euglenoidea 

 has recently been made by Dobell (3). In Copromonas subtilis, from 

 the intestine of the common frog and toad, reproduction is effected 

 by simple longitudinal fission accompanied by amitotic division of 

 the nucleus. After a period of from two to six days a considerable 

 number of individuals are found to be conjugating. All the 



