Furthermore, if we compare the protozoan as an individual, as a creature capable of living an independent life, with a many-celled worm or insect then without question the protozoan is much more easily regarded as the simpler, the less complex. But there is another way of approaching the question. We can as well compare the protozoan with the particular cells of the multicellular organ ism, and then we find that the protozoan leaves the cells of the higher animals far behind : the most complex known cells are found in the Protozoa.
For these and for various other reasons C. Dobell has pro nounced this comparison of the protozoan with a single cell of a many-celled organism to he essentially inadmissible, for by it we compare whole independent individuals (Protozoa) with depend ent parts (cells) of an individual. According to Dobell, the Pro tozoa are not unicellular but non-cellular organisms. This idea is not difficult to understand : if we compare the protozoan with the many-celled animal as a whole, we may say that the latter is divided into cells, the former not ; therefore the protozoan is not organized in a cellular manner. Nothing can be said against this conception in so far as it is limited to the physiological aspect, that is, to the functions of the individual; and yet it can easily be shown that the Protozoa are unicellular.
It has been mentioned above that in many protozoans we only find a single nucleus. Moreover this nucleus not only looks like the nucleus of any animal or plant cell, but also behaves in pre cisely the same way when the protozoan reproduces itself. This process generally takes the form of division ; one individual splits into two (or more) new individuals. The nucleus of the mother-cell also divides at the same time; this division of the nucleus proceeds like the nuclear division of a cell in animal tissue. It is therefore clear that, while from the physiological point of view we may compare the protozoan individual as a complete organism with a worm or a butterfly, from the structural and phys iological point of view (i.e., with reference to its organization) we must compare it with a single cell of the worm, etc.; for the protozoan does not multiply like a worm, but like a single cell of a worm.
The unicellular condition of many, but not all, protozoans may be demonstrated in yet another way. There are groups of Pro tozoa, such as the Volvocineae, which in clude unicellular and multicellular species.
The obvious close relationship between these single-celled and many-celled forms makes it quite clear that these latter are nothing but cell-colonies function ing as individuals. Let us compare
Chlamydomonas (fig. 1) with Eudorina (fig. 2) : the Chlamydomonas individual has a nucleus, a chromatophore and two flagella; when it reproduces it splits into two new independent individuals. The in dividual of Eudorina is made up of 32 cells, which have a perfectly regular arrangement inside an egg-shaped mucilage-sheath. When Eudorina reproduces, each of the 32 separate cells has to divide first into two ; each of these two divides again ; their products divide, and so on until after a series of five divisions each original cell of Eudorina has given rise to 32 cells which are held together in a common wall of mucilage (fig. 2b). At this point the gelatinous wall of the "mother colony" dissolves and each of the 32 new cell-groups slips out of the "skin" of its mother-cell, and begins an independent existence as a new Eudo rina individual. Now if we look more closely at a single cell of a Eudorina individual we find that it is exactly like an individual of Chlamydomonas (fig. ia). But the Eudorina colony has already reached a true multicellular condition ; its single cells, although like one another, are not capable of independent life. Yet we need not hesitate to compare them with the independent individuals of Chlamydomonas.
This example enables us to enlarge our original definition of Protozoa. We have said above that Protozoa are unicellular organisms, but to be more accurate we had better say "most Protozoa," because there are also these multicellular protozoan5 like Eudorina which are connected by transitional forms with single-celled species. There are also Protozoa that we cannot call unicellular (although they do not form cell colonies) because they contain numerous nuclei (fig. 24). But nearly all these polyenergid forms, as these multicellular and multinuclear protozoans are called, in contrast to the truly uninuclear and unicellular, mono energid forms, differ essentially from multicellular animals : there is no difference amongst the single cells, or nuclei, as the case may be, of such a polyenergid protozoan ; they are all approxi mately equal in regard to structure, as well as function. This difference between protozoans and higher multicellular organisms has been expressed by the terms homoplastic and heteroplastic. The multicellular or multinucleate protozoans, composed of sim ilar cells, are designated as homoplastic ; multicellular higher ani mals and plants, on the contrary, composed of unlike cells, are heteroplastic.