THALLOPHYTA 315 
It seems reasonable to suppose that the most primitive 
ancestors of the higher plants lacked a definite nucleus, and also 
that they did not have a method of sexual reproduction. More- 
over, it would seem highly probable that the first plant that 
possessed chlorophyll did not have definite chloroplastids. It 
appears, therefore, that the Cyanophyceae possess many features 
that must have been characteristic of some of the primitive an- 
cestors of the higher plants. While there is no evidence that 
the Cyanophyceae themselves gave rise to any higher group, it 
does seem probable that they and the higher plants had at least 
a common ancestor, and that in many respects the Cyanophyceae 
resemble this common ancestor very much more than do the 
higher plants. 
The only close relationship of the Cyanophyceae is to the bac- 
teria, which they resemble in lacking sexuality and a highly 
organized nucleus and in the method of cell division. 
CLASS FLAGELLATA 
The flagellates consist of unicellular or colonial aquatic or- 
ganisms which have both animal and plant characteristics. The 
name Flagellata refers to one or two (rarely more) slender, hair- 
like projections called flagella, the movement of which enables 
the individuals to swim in water (Fig. 317). 
Cell structure. The cell is naked or has a distinct membrane 
which seldom contains cellulose. Within a cell is a single nu- 
cleus and a pulsating vacuole, while many species have a red 
eyespot (Fig. 317). Some forms have well-developed green, 
yellow, or brown plastids which enable them to carry on photo- 
synthesis; others are colorless and live by absorbing organic 
matter from the surrounding water; while many can take in 
and digest solid particles. Some forms contain chlorophyll and 
‘manufacture food by photosynthesis when living in the light 
(Fig. 318), but when growing in the dark in nutrient solutions 
they lose their chlorophyll and absorb organic food from the 
surrounding medium (Fig. 318). 
