Thus, to begin with, the animal pedigree is divided into two very distinct grades, the Protozoa and the Metazoa.
We have analogies to this in the two nuclei of some of the protozoa, the one being solely for the purpose of propagation, the other being associated with the functional activities of the cell.
The old criterion by which they and other Metazoa were once distinguished from Protozoa, namely, the differentiation of large and small sexual cells from each other and from the remaining cells of the body, has been broken down by the discovery of numerous cases of such differentiation among Protozoa.
Nereid worms were abundant and interesting protozoa as well.
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.