310 Causes and Course of Organic Evolution 



Volvocacepe like Sphcerella and ChlamydomonaSy and thence 

 through alHes of Coccomonas and Chlorogonium to Stephano- 

 sphcera and Volvox. As in Endosphcera the chromatophore 

 forms a com])lete postero-Uiteral and peripheral investment 

 to the cell, in which one to several pyrenoids occur. 



A striking morpho-physiological structure that becomes 

 highly differentiated in this group is the red spot or eye-spot, 

 that attains most perfect condition in some of the highest 

 \ olvocaceae. 



Twenty of the twenty-two genera are fresh- water. The 

 living species number 31, 27 of which are fresh-water, and some 

 of these are world-wide in distribution. 



Another diverging evolutionary line that seems early to 

 have originated ^^'ith the three above named, but rather from 

 a greenish yellow or green-bro^ii cyanophyceous ancestry 

 like Sytiechococcus, includes the Gymnodiniacese, Prorocentra- 

 cese, Peridiniacese, and Diatomacese. Of these the first is 

 simplest, and includes genera like Amphidinium with green 

 or greenish bro\\Ti chromatophores, and so may have arisen 

 ^^'ith pleurococcaceous organisms from a common or nearly 

 allied ancestral cyanophyceous group, through perfected evo- 

 lution of a nuclear body. The greener species are still fresh- 

 water, but others are now yellow-bro^n and have migrated 

 into a free marine environment where, though few in species, 

 they now are often abundant in indi\dduals. But still others 

 like Gyinnodinium and Cochlodiniiim show all transitions from 

 green through yellow-bro^^^l to pale yellow and finally color- 

 less conditions, while they pass from fresh- water to salt-water 

 life. Such colorless flagellate free-s\\'iimning organisms have 

 often been claimed as protozoic animals by the zoologist. 

 They rather should be viewed as a small side-line of plant life, 

 the species of which have passed from typical plant life to 

 types that simulate animal form and relation. 



But while in their more primitive stages of development, 

 and while still retaining a single or later a divided bro^Ti chro- 

 matophore, types somewhat intermediate between Hemidinium 

 and Pyrocystis of the Gymnodiniacese showed deepening of 

 the longitudinal furrow and thickening of the cellulose mem- 

 brane, at the same time that minute pore areas were left in 

 the latter. Some of these, developing and perfecting one or 

 two flagella, seem to have branched out into the Peridiniaceae 

 that includes three genera and twelve species of fresh- water 

 and 21 genera with 105 species of marine distribution; also 

 the Prorocentracese that is now wholly marine in its three 

 genera and ten species. 



