What we have just said still does not apply to all Protozoa; it is, indeed, almost characteristic of living organisms that they do not admit of neat classification. There are a few Protozoa that are heteroplastic (Voivox, the Myxosporidia, etc.). The colonies of Volvox (fig. 19), a protozoan closely allied to Eudorina, are made up of 12,000 to 22,000 cells, but only a small number of these are capable of developing into new colonies ; most of the cells are incapable of this and degenerate and die when the old colony gives rise to new ones ; these cells are only capable of vege tative functions of movement and nutrition, whereas the repro ductive cells are non-motile. Thus in Volvox we have already arrived at a division of labour between vegetative body- and repro ductive germ-cell.
If in spite of this we number these forms amongst the Protozoa it is either because, like Volvox, they are linked by a series of transitional forms to typical unicellular species, or because, like the Myxosporidia, they are separated by their lack of proper organs from the lowest multicellular animals, the sponges. On the other hand the division between the Protozoa and the lower plants (Algae and Fungi) can only be arbitrary and therefore difficult to determine.' We may give as an illustration of this the fact that there is a long-standing dispute between botanists and zoologists as to whether certain forms, such as the Volvocineae, should belong to the Protozoa or to the Algae ; for this group has undoubted affinities with true multicellular plants (the green algae). But we will not concern ourselves with these difficulties here. We need only emphasize the fact that we know of no single protozoan type that could be looked upon as the ancestral form of any known multicellular organism.
We can divide the Protozoa into numerous groups (orders or classes) which we may call natural, i.e., protozoans belonging to one of these groups appear to be related as the dog is to the bear and the cat. But it is only rarely that we can trace between the various groups of Protozoa a relationship like that which exists between the mammals and the birds. Many protozoans are as little related to one another as the mammals are to the insects. On this account it is naturally difficult to make generalizations about anatomical and other similar characters which will hold for all Protozoa. A cursory glance at the text figures will show that the form can be extremely diverse. Some are completely shape
less, others rigidly constructed; some are asymmetric, and others completely symmetrical. A comparison of the magnifications given in the figures shows further that the absolute size can be immensely variable. There are protozoans that only measure about in. and others some thousand times this diameter; indeed there are some giants, like Nummulites (a foraminiferan) which are as big as a shilling piece, and the plasmodia of many myxomycetes are many times larger.
'All protozoans are marked off from the Bacteria by the possession of a typical cell-nucleus.
In considering the anatomical form of the Protozoa we have already mentioned that the protozoan always consists of a cell body (cytoplasm) and cell-nucleus (or "nucleus" for short). Cell body and nucleus are composed of that mysterious substance we call protoplasm (q.v.), which is the very material of life.
We can often study the structure of protozoans thoroughly enough in the living animal. As a rule, however, for the purpose of making more complete observations we make use of two tech nical processes that are indispensable in work on the microscopic anatomy of the higher animals and plants. First we clot, or coagu late, the protoplasm with various chemical reagents, just as we coagulate the albumen of an egg by boiling it ; we then soak it with various dyes; the individual constituents of the cell-body have now, in their coagulated condition, the property of absorbing these dyes and they have moreover particular preferences for cer tain colouring materials (see CYTOLOGY) ; making use of this we can stain different parts of the cell different colours, and so make them more clearly visible.