Protozoa

Nucleus.

The nucleus of Protozoa is a clearly defined proto plasmic body, usually of rounded shape although there are other forms; as a rule only the so-called nucleolus, a small spherical body of albuminous substance whose structure and function are not as yet exactly understood, stains differently from the rest of the nucleus and can be distinguished from the main body of the nucleus proper, the karyoplasm. Nucleoli which are very big in comparison with the rest of the nucleus and are only present sin gly are called karyosomes. The surrounding karyoplasm may show various kinds of structure, granular, spongy, thread-like, or it may appear homogeneous.

It is with regard to the function of the nucleus that the Proto zoa have provided some interesting facts; it is easy enough to cut in two a large uninucleate proto zoan such as Amoeba, and if we watch the later behaviour of the two pieces, one of which is of course without a nucleus, we find that only the part with the nucleus can develop again into a complete individual ; the part without a nucleus, on the other hand, although it can remain alive for a little while and even move about, is quite unable to take in and digest food ; above all it is incapable of developing into a complete individual and sooner or later it consequently degener ates and dies (fig. 3). A nucleus can only come from a nucleus; it cannot arise de novo from the cytoplasm. The nucleus is there fore an organ essential for the life of the cell; we cannot yet define its functions in the Proto zoa more exactly. Investigations of inheritance in the higher plants and animals have shown that it is the seat of the hereditary material and this is no doubt also the case with the Protozoa (see HEREDITY).

We have already shown that many protozoans are multinucleate, and in these forms the nuclei are usually all alike. Yet there are Protozoa with dissimilar nuclei. These are the Ciliata, possessing in the simplest case two nuclei (fig. 4), the so-called macronucleus and the much smaller micronucleus. We shall discuss the impor tance of this "nuclear dimorphism" later.

Organoids.

Cytoplasm and nucleus are the only body-parts that all Protozoa have in common' ; of all other organs we can only be sure that they occur in certain species. We shall therefore deal with these structures quite briefly. A distinction can be made between organoids and non-protoplasmic inclusions. The organoid.

e.g., a chromatophore, cilium or myoneme contractile element, is composed of living protoplasm which is more or less clearly delimited from the cytoplasm and performs a particular function, and comparable in one way therefore with an ordinary organ of a multicellular animal such as stomach, eye or muscle. Non-proto

plasmic inclusions, on the other hand, such as a membrane or a siliceous skeleton, are non-living, like a snail's shell.

One of the most impor tant organoids—indeed we may say in a certain sense the most important organoid—is the chloroplast or chromatophore, which we only meet with in those protozoans which feed like green plants. The chromato phore is sharply marked off from the cytoplasm and consists of protoplasm saturated with chlorophyll, the green colouring matter of leaves (fig. 1) ; like the nucleus it can only arise from division of another chrom atophore.

There are several kinds of such cell-constituents capable of division. Amongst these are to be included the centrosornes which play such an important part in nuclear division; these are spheri cal and for the most part they are rather small structures. Closely related organically to the centrosome is the basal granule, a nodular thickening at the base of a flagellum or cilium (figs. I, 16, I 7 ; for further discussion, see below).

Most of the organellae devel oped in the protozoan cell are however incapable of division; that is, they arise usually out of the undifferentiated cytoplasm and can disappear again into it. To this group belong the static fibrillae, more or less stiff threads or little bristles which serve as an internal skeleton for many Protozoa. They thus ensure the maintenance of the typical body form of the organism much as the metal framework keeps an 'Whether also the so-called mitochondria and Golgi-bodies (see CYTOLOGY: Cell inclusions) are present in all protozoans is not yet known ; they have only so far been found in a few species.

umbrella in shape (fig. ). In all probability such static fibrillae take part in the formation of flagella and cilia. We speak of flagella when these are long, as compared with the body itself, and few in number (figs. 7, 16). Cilia are relatively short and numerous (figs. 4, 8). Cirri are nail-shaped structures which consist of several cilia stuck together. These hair- or string-like appendages of the body have a locomotive function in many protozoans and in all probability always consist of a rigid axis (static fibril) surrounded by a relatively less viscous mantle of protoplasm. The rigid axis arises from a basal granule which is probably to be regarded as its centre of f ormation; many protozoans can be deprived of their cilia—shaved, as it were—and then the basal granule can be seen to grow a fresh cilium (fig. 8).