Protozoa

A third kind of fibrillar organ oid is the myoneme, a contractile fibre or band which is the muscle of the protozoan ; very little is known of the structure of this ele ment except that, in most cases at least, it corresponds to the non-striated rather than to the striated muscle of the higher ani mals (see MUSCULAR SYSTEM).

We do not know definitely whether there are in some Proto zoa, as many investigators have claimed, apart from the fibrillar organoids just mentioned, fibres which serve, like the nerves of the higher animal, for the con veyance of sensory impulses.

No more can we be certain of true sense-organs in the Protozoa, although the red granule, or stig ma, found at the fore-end of all green protozoans is very fre quently considered as analogous to an eye. Its sensitiveness to light has not yet, however, been critically demonstrated, although it is quite possible that it repre sents a sense-organ (for a more detailed discussion see below).

Another organoid, found in protozoans living in fresh water, but not as a rule amongst those living in the sea or as parasites, is the contractile vacuole. This may be called the kidney of the protozoan. In the simplest cases it is merely a bubble or bladder which pours out its contents at intervals through a very minute opening. The lining of this bubble is marked out by special properties which we need not go into more deeply here. The con tractile vacuoles of many protozoans are much more complicated and consist usually of a chief vacuole, emptying itself directly into the exterior, and a system of subordinate or accessory vacuoles emptying themselves into the central vacuole. The accessory vacuoles therefore work in alternation with the central vacuole: when the accessories empty the central one swells and when it empties the accessories fill up again (fig. 4).

Up to now we have only been dealing with relatively simple organoids; we meet a more complicated type in the cytostome, the mouth of some protozoans. Many protozoans have no mouth at all and yet are able to take in food (see below, Nutrition) but others have a proper mouth. In the simplest cases it is nothing but a trough or slit-like deepening of the body surface (fig. 1). In more complex forms the cytostome is a cavity held open by supporting fibrillae (fig. 8) ; it is moreover carpeted with cilia, which drive the food in a current into the blind end of the cyto stome (fig. 4). Unlike the mouth of a higher animal, the cyto stome is not continued in a stomach but comes to a blind end. The food is driven against the blind end of the cytostome, which then swells out until a so-called food-vacuole is cut off from it ; this is pushed into the inside of the cell-body and the food there digested (fig. 4).

Many protozoans have also a cell anus or cytoproct which usually consists of a mere hole held open by supporting fibrillae.

Non-protoplasmic Bodies.

Amongst the by-products of the protoplasm, which we shall call alloplastic bodies, are the skin and the various kinds of protective covering or armour. Some Proto zoa are completely naked, i.e., their surface consists of ordinary protoplasm. With other forms, the surface, though still living protoplasm, is hardened into a more rigid skin (pellicula). With still others the protoplasm differentiates on its surface a non living, more or less rigid coat, and this coat is often separated by a space from the body of the cell. Nothing exact is known of the chemical composition of these skins ; possibly they are often of a material related to the chitin of an insect's skin; in many green protozoans the skin consists of cellulose, as in plants, but fre quently it is not rigid at all but merely a slime-coat (fig. 2). Finally by the deposition of mineral matter such as chalk or silica some protozoans change their skins into a protective shell or armour, in the same way that a lobster's shell is constructed by deposition of chalk in a ground mass of chitin. Some protozoans, for example the Euglypha, also secrete mineral substances inside their bodies for later use in building up a hard skeleton.

We need not discuss here all the other non-living substances that we meet with in the bodies of Protozoa (starch, fat, etc.) ; as far as we understand their role they seem to act as reserve intergrades between the unicellular and uninucleate forms and the multicellular and multinucleate. A protozoan that almost always has four nuclei we can still call unicellular; a plasmodium of a myxomycete (slime mould), whose thousands of nuclei are embedded in a single mass of protoplasm, scarcely has a title to this name; yet we can hardly call it multicellular, so the name "polyenergid" has been invented for this kind of creature. The gap between one-celled and many-celled Protozoa is filled by Protozoa that form colonies ; when the progeny to which a proto zoan has given rise by division stay together, the aggregate is called a colony. Polyenergid Protozoa arise from uninucleate germ-cells in a similar way, but only the nucleus divides while the cytoplasm remains undivided. When the individual cells of a colony are arranged in a particular order and especially when they are no longer capable of independent existence we have arrived at the stage at which we can call the colony a multicellular individual.