PROTOCOCCOIDE/E 409 



Class XXV. Protococcoideae. 



In this class, the Chlorophyllophyceae of some writers, are included 

 those simplest forms of vegetable life in which the endochrome consists 

 of pure chlorophyll of its natural green colour, sometimes replaced, 

 to a larger or smaller extent, by a red pigment, but the cell-sap never 

 pervaded, as in the Cyanophycese, by a soluble blue colouring-matter. 

 The individuals are of microscopic size, and may be either motile or 

 resting, and very commonly the same species occurs in both conditions. 

 The motile or protococcus form is, in the lower members, strictly unicel- 

 lular, consisting of chlorophyllous protoplasm either naked or invested 

 with a very delicate coat of cellulose or of a carbohydrate nearly allied 

 to cellulose, usually developing but little or no mucilage, and moving 

 freely through the water by means of a pulsating vacuole and two vibra- 

 tile cilia. In the resting condition the individuals are invested by a 

 much thicker cell-wall, and have a tendency to congregate or coalesce 

 i n to palmelloid families, and to enclose themselves in a common gela- 

 tinous envelope. In this state they multiply rapidly by repeated bipar- 

 tition. The palmelloid form may be derived directly from the proto- 

 coccoid, the protococcus-cells coming to rest, losing their cilia, and 

 investing themselves with a thicker cell-wall of cellulose ; or, in the 

 higher members, the individual consists of a number of gonids, chloro- 

 phyllous masses of protoplasm, enclosed in a common watery hyaline 

 envelope of mucilage, and propagation takes place by the escape of 

 these gonids from the envelope in the form of naked biciliated zoospores 

 or swarm-spores, closely resembling protococcus-cells, which, after going 

 through a motile period, come to rest, lose their cilia, invest themselves 

 with a coat of cellulose, and multiply by repeated bipartition in the 

 palmelloid form. In some cases these swarrn-spores are of two kinds, the 

 smaller ones being conjugating zoogametes. In no case is the individual 

 filiform and divided by transverse septa, as in the higher families of 

 the Cyanophyceae. 



It cannot be too strongly insisted on that this class is a purely pro- 

 visional one. Many of the forms at present included in it are, in all 

 probability, nothing but stages in the development of algae of consider- 

 ably greater complexity of structure belonging to widely separated 

 families. The external resemblance between the Protococcaceae and the 

 Chroococcaceae, and 'the parallel series of forms in these two families, 

 does not probably represent any genetic affinity. There is, on the other 

 hand, an undoubted alliance with the Pandorineae, through Chlamy- 

 dococcus and Chlamydomonas, as well as with the Hydrodictyese and 



