CHAPTER X 

 THE VOLVOCINJE 



THE three genera, Pandorina, Eudorina, and Volvox, are of 



special interest because they form a progressive series of 

 increasing complexity both in their organisation and in their 

 mode of reproduction. All three are found in stagnant water. 

 Whereas in the Protozoa which we have hitherto studied 

 the products of cell division separate sooner or later from one 

 another and lead a free and independent existence as separate 

 cells, or, as is the case in Badhamia, the cells after a short 

 period of separate existence fuse together and form a plas- 

 modium in which the cell outlines are no longer distinguishable, 

 we find in the Volvocinae that a certain number of the cells 

 remain adherent, and form a cell-colony. 



One of the simpler forms of these colonies is illustrated 

 by Pandorina morum. The unit of organisation is an 

 ovoid cell, much resembling Polytoma in form, since it is 

 furnished with a pair of flagella of equal length, a mem- 

 branous envelope, a contractile vacuole, a stigma composed 

 of haematochrome, and a vesicular nucleus. It differs, 

 however, from Polytoma in possessing a large chromato- 

 phor coloured green with chlorophyll. These units, in- 

 stead of living separately, are aggregated together to form a 

 simple colony composed of sixteen, more rarely of thirty-two, 

 similar individual cells. The colony is spherical or oval 

 (fig. 44, A) ; the cells composing it are wedge-shaped, their 

 broader ends external, their pointed ends meeting together in 

 the centre of the colony. Each cell has its own membranous 

 envelope, but in addition there is a colonial envelope of like 

 structure and composition in which the individual com- 

 ponents are embedded. The colony swims about by the 

 united action of the flagella of its members, and its nutrition 

 is holophytic. 



Multiplication of the colony is effected in two ways. The 



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