The four principal phases in the development are: (I) Blastula, (2) Gastrula, (3) Flagellate Embryo, (4) Larva.
No unicellular Rhodophyceae are known, although a flagellate organism, Rhodomonas, has recently been described as possessed of the same red colouring matter.
Similarly, while Diatomaceae may be excluded from among Phaeophyceae, though retained among algae, the Cryptomonadaceae and Peridiniaceae, like Euglena and other Chlorophyceae, may be excluded from Thallophyta and ranged among the flagellate Protozoa.
The plankton is divided into (a) the Zoo-plankton (such as the minute crustacea and the eggs and larva of fishes and many other marine animals); and (b) the Phyto-plankton, that is, the minute algae, diatoms, peridinians, some flagellate protozoa, spores of alga, etc. The investigation of the plankton from a new point of view, begun by Hansen in 1889, was continued by Lohmann at Kiel, by Cleve in Sweden, by Gran and Ostenfeldt in Norway and Denmark, and by Herdman, Allen and others in England.
A, Ordinary forms of varying sizes; b c, stages in multiple division; d, binary fission; e, f, g, flagellate forms (after Rogers).
The existence of ciliated micrococci together with the formation of endospores - structures not known in the Cyanophyceae - reminds us of the flagellate Protozoa, e.g.