244 SPH./ERELLA 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), composed 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. Obviously 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 Sphserella, 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 com- 

 pound. 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, loses its flagella, and 

 forms a thick cell-wall of cellulose (Fig. 68, B), thus 

 becoming encysted. So that, as in Amoeba, there is an 



