268 



FRESH-WATER BIOLOGY 



193 (192) Colony flattened, horseshoe-shaped, with poles differentiated by 

 arrangement of cells. Tails at posterior end. 



Platydorina Kofoid. 

 Representative species. . . Platydorina caudata Kofoid 1899. 



Colony slightly twisted in a left spiral. Cells sixteen or 

 thirty-two imbedded in a transparent, gelatinous matrix and 

 surrounded by a distinct sheath. 



Each cell has two flagella, an eye-spot, a nucleus, and a 

 single chromatophore. Tails, in sixteen cell colonies, are 

 three in number; in thirty-two cell colonies five tails are 

 present. Movement by rotation on the longitudinal axis. 

 Length 150 ^u. Plankton of rivers and lakes. 



Fig. 449. Platydorina caudata. X 185. (After Kofoid.) 



194 (191) Colonies spherical or ellipsoidal; cells differentiated as to size and 



function. . ... ... 195 



195 (198) No protoplasmic processes connecting the cells. Small vegetative 



cells at the anterior pole, large gonidial cells at the posterior 

 pole. Pleodorina Shaw 196 



196 (197) Cells sixty-four or one hundred and twenty-eight, about equally 

 divided between large and small. 



Pleodorina calif ornica Shaw 1893. 



Colony spherical, with gonidial cells two or three 

 times the size of the vegetative cells. Cells biflagel- 

 late, not in contact with each other. 



Reproduction asexual, by gonidial cells, in this and 

 other species of the genus. 



Found in ponds, ditches, and streams. 



Fig. 450. Pleodorina calijornica. X 300. (After Shaw.) 



197 (196) Cells thirty-two, rarely sixteen or sixty-four. Vegetative cells, 

 four in number. . . Pleodorina illinoisensis Kofoid 1898. 



Colony ellipsoidal with cells arranged in five circles; 

 the polar circles with four cells each, the other three 

 circles with eight cells each. The gelatinous sheath 

 enclosing the colony is of two layers. 



Gonidial cells much larger than vegetative cells, the 

 latter always directed forward during movement. 



Each cell with two flagella, an eye-spot, a nucleus, 

 and a single chromatophore. 



Average length 113 11. Plankton of rivers. 



Fig. 4S1. Pleodorina illinoisensis. X 200. (After Kofoid.) 



