THALLOPHYTA 



373 



which represents approximately the source from which they may 

 have originated. It is a very general opinion that such a source 

 is to be found among the Flagellatae, a family which it is 

 difficult to refer definitely either to the Kingdom of Animals or of 

 Plants. It includes many of those organisms which cause certain 

 diseases in man and other animals, and these are more definitely 

 animal in their characters. But others, such as Euglena, possess 



FIG. 314. 



Euglena gracilis. A, Form with green chromatophores (ch) ; n= nucleus; v = 

 vacuole and red eye-spot; g=flagellum. B=hemi-saprophytic form with small 

 green chromatophores. C colourless saprophytic form occurring in nutrient 

 solutions, in absence of light. D resting cyst of the form C : r=red eye-spot. 

 E = germination of the resting cyst of the form A , by division into four daughter cells, 

 which later escape. (After Zumstein.) (A, Cx 630; Bx 650 ; D, Exiooo.) 

 (From Strasburger.) 



features characteristic rather of Plants. Euglena is found commonly 

 in summer, colouring the foul water that drains from manure heaps 

 a bright green. The organism is then seen in the motile state, as a 

 free-swimming, naked protoplast of elongated form, towed along by 

 a single flagellum (Fig. 314). There is a central nucleus (ri), several 

 green-coloured chromatophores which vary according to the con- 

 ditions (ch), a contractile vacuole (v.) communicating by an oesophagus 

 or funnel with the exterior, and a red eye-spot, or stigma, lying at 

 the junction of oesophagus and vacuole. The base of the flagellum 



