We next come to forms of fungus, which set up a process very like putrefaction, such as the yeast plant, Torula cerevisia, which causes alcoholic fermentation in sugar solutions. In the torula an external case containing protoplasm may readily be seen, and multiplication of the cells goes on rapidly by a process of budding. Torulae, however, like bacteria, though called vegetables, have not the power of assimilating as ordinary green plants do, but require nutriment to be supplied to them which already contains organic or complex compounds. Structurally but little different from torula is a one-celled plant, the green protococcus, which, like a higher plant, can build up its texture from the simplest food stuffs, and carry on its functions. It consists of a case made of cellulose, within which lies a mass of protoplasm with a nucleus. Their protoplasm is colored green by a peculiar substance called chlorophyll. We shall see presently that it is to protoplasm containing chlorophyll that plants owe all their most characteristic and wonderful properties; viz., the property of assimilating so as to construct complex carbon compounds out of simple inorganic materials.

Two different forms of Amoeba; in different phases of movement.

Fig. 40. Two different forms of Amoeba; in different phases of movement. Those on the left (after Cadiat.) A and B show an outer clear zone. (Gegenbauer).

The smallest and simplest organisms classed as animals are generally larger than the vegetable cells just alluded to. They consist of protoplasm without any nucleus, and only sometimes with a structural difference between any part of their substance. As an example we may take Protamoeba. This is a small mass of protoplasm without any nucleus, but its outer layer is clearer and less granular than the central part. It can move by sending out protoplasmic processes, in which currents can be observed resembling those in the vegetable cells. Excepting as regards the nucleus, it is much the same as the Amoeba, which can be readily found and watched, and will be more accurately described.

The amoeba is a single cell or mass of uncovered protoplasm, containing a well-defined nucleus, within which is a nucleolus. There is also generally a vacuole. The central part of the protoplasm is densely packed with coarse granules, but the outer, more active part is structureless and translucent looking, somewhat like a fine border of muffed glass, encasing the coarsely granular middle portion. Such an animal has no parts differentiated for special purposes, the requirements of its functions being so limited that the protoplasm itself can accomplish them.

Thus the processes of protoplasm, which flow out with considerable rapidity from the body, frequently encircle particles of nutrient material, and then closing in around them, press them into the midst of the granular central mass. Here they sojourn some time, and during this period no doubt any nutritive properties they possess are extracted from them, and they are then ejected from the plastic substance. This form of assimilation demands no previous preparation of the food such as we shall see takes place in the alimentary tract of man, and in the special organs of the higher animals; yet it is a form of digestion adequate at least to the requirements of this simple organism. The repeated alteration of relationship between the different parts of the protoplasm, and the surrounding medium during the flowing hither and thither of the currents, produces not only a change in the shape and position of the animal, but also acts as a means of distributing the nutriment to the different parts of the body, and of collecting and carrying to the surface the various products of tissue decomposition; thus the streaming protoplasm does the work of a circulating fluid such as we see in the more elaborate organisms for the distribution of nutriment and elimination of waste materials. The surface of the amoeba is sufficient to allow of the gas interchange necessary for life, and by means of the ever-changing material exposed, sufiicent oxygen is taken for its tissue combustions, and so a function of respiration is established. The growth that results from the perfect performance of these vegetative functions proceeds until the maximum size is attained, and further nutritive activity is then devoted to reproduction. When growth ceases, commonly the cell divides and forms two distinct individuals. The movements which form the most striking operations of the amoeba are the same as those which take place in protoplasm, except that they are more rapid and obvious. The clear, outer layer first flows out as a bud-like process, and, as it is gradually enlarging, some of the central granular part of the cell suddenly tumbles into its midst, where it remains, while other pseudopodia are being thrown out in the neighborhood, and the same changes repeated in them. It is difficult to watch the motions of an amoeba without being impressed with the idea that it is not only endowed with sensibility, but that it can also discriminate between different objects, for we see it greedily flowing around some food material, while it carefully avoids other substances with which it comes in contact.

If a glass vessel containing several amoebae be placed in a window, they will be found to cluster on the side of the glass most exposed to the light. From this it would appear that, in some obscure way, protoplasm can appreciate light, and respond to its influence by moving toward it.

This single-celled animal, or nucleated mass of protoplasm, can perform all the functions of a higher animal. It can move from place to place and assimilate nutriment, apparently discriminating between different materials. It distributes nutrient stuffs and oxygen throughout its body by a kind of tissue circulation, and it can appreciate and respond to the most delicate form of stimulus, namely, light, which subtle motion has no effect on the sensory nerve fibres of the higher animals.

In some unicellular animals certain parts of the cell are specially modified for the performance of special functions, a division of labor thus taking place which insures the more perfect accomplishment of the different kinds of activity. In one of the commonest of the Infusoria (Paramoecia bursaria), which swarm in dirty water, this is well exemplified. The outer layer of the flattened body is denser, and forms a kind of fibrillated corticular case (ectosarc), which is covered over with hair-like processes (vibratile cilia), constantly moving in a certain direction, so as to propel the creature rapidly through the water. The internal part of the cell is very soft, almost fluid, and coarsely granular in appearance, containing many bodies which have obviously been introduced from without. This soft internal protoplasm (endosarc) moves slowly round in a definite direction, completing its circuit in one or two minutes, and thus carries on a circulation which mixes the various matters contained in it. At one point of the ectosarc, or cortical layer, an orifice or mouth leading to an oesophageal depression is found. This orifice is lined by moving cilia, which by their vibrations drive the food into the oesophagus, whence it is periodically jerked into the soft internal protoplasm or endosarc, together with some water, and thus forms a food vacuole, which is carried round in the circulation of the ectosarc. Besides a well-marked nucleus and nucleolus in the central part of the cell, these paramoecia have one or more clear spaces placed near the surface at the extremities of the animal. These vacuoles suddenly contract, and disappear every now and then. When this contraction occurs, fine canals radiating from the contractile vacuole are distended with the clear fluid which has probably entered the vacuole from without. Thus a permanent set of water vessels carry fluid from the contractile vacuole throughout the endosarc.

Diagram of Para moecium showing digestive cavity.

Fig. 41. Diagram of Para-moecium showing digestive cavity. (a a) Body space filled with soft protoplasm, into which food is taken.(b) Mouth. (c) Anus. (d) Contractile vesicle. {After Lach-mann).

In such an animal there is a distinct advance of function compared with the amoeba; a more elaborate and specialized method of feeding; a more systematic and regular circulation of nutrient matters; a respiratory distribution of water by the contractile vesicle and its water canals; more rapid motion; and more obvious sensation.

In the bell animalcule, or Vorticella, the same kind of division of labor exists, but in one of its commonest conditions it is attached by a thin stalk to the stalk of some weed or other object. Besides the ciliary movement we here find that the general mass of the protoplasm can suddenly and forcibly contract, so as to completely alter its shape, and change the bell into a rounded mass. This spasm of the body is commonly associated with a wonderfully rapid contraction of the stalk. This stalk consists of a delicate transparent sheath, in the centre of which is a thin thread of pale protoplasm. The rapid contraction of the protoplasm of the stalk and the spasm of the bell occur on the application of the least mechanical excitation, such as a touch to the cover glass. Here in a single cell we have certain portions set apart for special purposes, most of which are the same as in paramoecia. But the animal being attached requires a special way of escaping from its enemies, and hence we find it endowed with three special forms of motion. Besides the ciliary and streaming protoplasmic motion, its body can spasmodically change its shape, and the stalk contracts with a velocity comparable with that of the most specifically modified contractile tissue (muscle) of the higher animals, by means of which their rapid and varied movements are carried out.