PROTOPHYTA 



53 



Class Isokontae 



All the Isokontae* are plant-like forms in which the chloro- 

 phyll is green and made up of two green and two yellow pig- 

 ments in approximately 

 the same proportions as 

 in higher plants. True 

 starch is found as a char- 

 acteristic product of nu- 

 trition, and cellulose is 

 a main constituent of the 

 cell wall. The forms in- 

 cluded here are widely 

 distributed in fresh 

 water ponds and pools 

 but rarely occur in salt 

 water. Theoretically, 

 they are interesting in 

 that, more than any 

 other group of algae, 

 they are regarded as 

 standing in the direct 

 line of evolution of the 

 higher plants; in other 

 words, they are consid- 

 ered to be related to an- 

 cestral forms of the pres- 

 ent day higher groups. 



The simplest and 



smallest of these forms 



are freely motile (Fig. 



25) and the palmella 



phase is either absent or 



temporary, but colony 



formation is very COm- 

 mnn nnri rhp rnlnnlpc i rp They are highly modified dinoflagellates, one type. A, 

 mon anU me COlOmeS are rec ^ ling the structure of filamentous algae, the other, 



motile. In some of the 

 larger and related types 



Fig. 24— HAPLOZOON CLYMENELLAE 

 Parasites of the digestive tract of a marine worm. 



B, the plate-forming habit of thallophytes 



From photomicrographs by the author 

 Magnification, 170 



* The term Isokontae refers to the equal-size flagella or motile organs on the 

 gametes. 



