II CELL-WALL 27 



organism. These are small ovoidal structures (A, 

 with clearly defined outlines occurring in varying numbers 

 in the chromatophores. When treated with iodine they 

 assume a deep, apparently black but really dark blue, 

 colour. The assumption of a blue colour with iodine is the 

 characteristic test of the well-known substance starch, as 

 can be seen by letting a few drops of a weak solution of 

 iodine fall upon some ordinary washing starch. The bodies 

 in question have been found to consist of a proteid substance 

 covered with a layer of starch, and are called pyrenoids. 

 Starch itself is a definite chemical compound belonging 

 to the group of carbo-hydrates, i.e., bodies containing the 

 elements carbon, hydrogen, and oxygen : its formula is 



C 6 H 10 5" 



In Hsematococcus pluvialis there is no contractile vacuole, 

 but in another species, H. laciistris, this structure is pre- 

 sent as a minute space near the anterior or pointed end 

 (Fig. 3, E, c. vac.}. 



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 around the green body, but 

 by careful focusing is seen to be really an extremely thin 

 globular shell (A, c.w.) composed of some colourless trans- 

 parent 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. i, 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 Amceba, as stated in 

 the preceding lesson (p. n) is very probably nitrogenous : 



