124 PLANT-ANIMALS [CH. 



essentials, the first green cells which appear in the 

 body of C. roscoffensis. The bulk of the cell (Fig. 21) 

 is occupied by a flask-shaped chloroplast (Chi.} in the 

 middle of which lies the pyrenoid surrounded by its 

 starch sheath. In the "neck 5 of the flask-shaped 

 cell lies a colourless plug or core of protoplasm in 

 which the nucleus is suspended. On one side of the 

 chloroplast, a red eye-spot is placed (St.). So far, the 

 description of the active cell corresponds exactly 

 with that of one of the first green cells to be seen 

 in the body of the infected animal. But, in addition 

 to these structures, two others are met with in the 

 free, active green cell which are absent from the 

 green cells contained in the body of C. roscoffensis. 

 These new structures are flagella and cell- wall. 



The flagella (Fig. 21) consist of four equal, delicate 

 protoplasmic threads each about twice as long as the 

 green cell. They project from the anterior end of 

 the colourless plug of protoplasm, and by their active, 

 contractile movements serve to row the animal 

 through the water. The cell-wall which invests the 

 alga is extremely delicate and gives, when treated 

 with appropriate reagents, the reaction not of cellulose 

 but of chitin. 



The flagellated cells are remarkable in that they oc- 

 cur in two sizes (Fig. 21). The large green cells macro- 

 cytes are about twice as big as the small microcytes. 

 Such a difference in size occurs not infrequently among 

 unicellular green algse, and in cases where it occurs it 



