244 SPH.^RELLA CHAP. 



There is still another characteristic structure to which 

 no reference has yet been made. This appears at the 

 first view something like a delicate haze round the red 

 or green body, but by careful focussing is seen to be 

 really an extremely thin globular shell (A, c. w], com- 

 posed of some colourless transparent material, and 

 separated, by a space containing water, from the body, 

 to which it is connected by very delicate radiating 

 strands of protoplasm. It is perforated by two extremely 

 minute apertures for the passage of the flagella. Ob- 

 viously we may consider this shell as a cyst or 

 cell-wall, differing from that of an encysted Amoeba 

 (Fig. 67, D) in not being in close contact with the 

 protoplasm. 



A more important difference, however, lies in its 

 chemical composition. The cyst or cell-wall of Amoeba, 

 as stated in the preceding chapter (p. 232), is very 

 probably nitrogenous ; that of Sphaerella, on the other 

 hand, is formed of a carbohydrate called cellulose, allied 

 in composition to starch, sugar, and gum, and, like 

 starch, having the formula C 6 H 10 O 5 . Many vegetable 

 substances, such as cotton, consist of cellulose, and 

 wood is a modification of the same compound. Cellulose 

 is stained yellow by iodine, but iodine and sulphuric 

 acid together turn it blue, and a similar colour is 

 produced by a solution of iodine and potassium iodide in 

 zinc chloride known as Schulze's solution. These tests 

 are quite easily applied to Sphaerella : the protoplasm 

 stains a deep yellowish-brown, around which is seen a 

 sort of blue cloud, due to the stained and partly-dissolved 

 cell- wall. 



After leading an active existence for a longer or shorter 

 time, the Sphaerella comes to rest, losing its flagella, and 

 its protoplasm being closely surrounded by the cellulose 

 cell-wall (Fig. 68, B). So that, as in Amoeba, there is an 



