THE SIMPLER ORGANISMS 



107 



marginal cells (three on each side) with flagella. The cells 

 all have rather thick cellulose walls, and there is a 

 common gelatinous envelope investing the 

 entire colony. The motion is that of a 

 whirling disk. 



Pandorina (fig. 64) is a small spherical 

 colony of bi-flagellate cells. Usually there 

 are sixteen (sometimes only eight, more 

 rarely, thirty-two) of the cells, closely held 

 together in a gelatinous envelope. New 

 colonies are formed in two ways: i) repeat- 

 ed divisions within the old cell wall give 

 rise to new colonies, which escape as colo- 

 nies and not as single cells (fig. 64 i). 2) 

 each cell of a colony may divide four or five 

 times, giving rise to sixteen or to thirty - 

 two cells (sex cells, or gametes) which es- 

 cape singly and fuse in pairs, forming 

 zygotes (fig. 647). Each zygote, later, by 

 successive divisions gives rise to the normal 

 colony. 



Volvox is a spherical colony of somewhat 

 similar cells (fig. 65). It grows to large 

 size, being easily visible to the unaid- 

 ed eye, and it may contain hundreds or even thousands of 

 constituent cells, all embedded in the common gelatinous 

 matrix, their flagella radially protruded and lashing in uni- 

 son to produce a rolling motion. Volvox presents a remark- 

 able differentiation into vegetative and reproductive cells, 

 to be discussed under a subsequent heading. 



Study 15. A comparative examination of common flagellates. 



Materials needed: Living specimens of a variety of 



simple and colonial flagellates. Some of these will be 



FIG. 64. A colo- 

 nial flagellate 

 Pandorina. h, 

 a normal spheri- 

 cal colony; i, 

 daughter colon- 

 ies developing 

 within the cell 

 walls of the moth- 

 er colony ; j, 

 zygote resulting 

 from the fusion 

 of the gametes; 

 ,gamete; these 

 are often of un- 

 equal size. 



