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 œsophageal 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 paramecia 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.

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 paramœcia. 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.

CHAPTER V.

FOOD.

The continuation of life depends on certain chemical changes which are accompanied by a loss of substance on the part of the active tissues. This loss must be made good by the assimilation of material from without, and the manner in which assimilation takes place constitutes one great point of difference between Plants and Animals. In the majority of the former (certain fungi form the main exceptions) the cells in those portions of the plant which are exposed to the light and air contain a peculiar green substance called chlorophyll, and through the agency of this substance they are able to obtain from the inorganic kingdom nearly all the food they require. Water, with such salts as may happen to be in solution, is taken up by the roots, and carried through the stem to the leaves; here the active chlorophyllbearing cells, under the influence of the sun's rays, cause its elements to unite with the carbon dioxide present in the air, and form various substances, of which we may take starch or cellulose as an example. The reaction may be represented chemically, thus:

6CO2 + 5H2O = C6H10O5 + 012.

Starch or Cellulose.

A large proportion of oxygen is thus set free and discharged into the atmosphere.

The most striking property of plant protoplasm is the power of using the energy of the sun's rays to separate the elements of the very stable compounds, carbon dioxide and water, and from the elements thus obtained to make a series of more complex and unstable compounds, which readily unite with more oxygen, and change back to carbonic anhydride and water.

The carbon compounds made in and by the protoplasm of the green plants are some of the so-called "organic compounds,"

which enter into the composition of both plants and animals, and form an essential part of the food of the latter. They may be divided into three groups—

1. Carbohydrates-bodies so called from the presence of hydrogen and oxygen in proportion to form water; e.g. — Starch, CH10O5 = C¿(H2O)5.

Grape sugar (dextrose), CHO6 = Ce(H2O)6.

Cane sugar (sucrose), C12H22O11 = (C12H2O)11

2. Fats-compounds of carbon and hydrogen with a less proportion of oxygen than the starches, e. g. :—

Olein (principal constituent of olive oil), Cs,H10406. 3. Albuminous bodies which contain nitrogen in addition to carbon, hydrogen and oxygen. These are of complex composition, and, as a rule, cannot be represented by chemical formulæ.

Animals cannot thrive on the simple forms of food obtainable from the inorganic kingdom, which suffice for the nutrition of a plant. They require for assimilation materials nearly allied in chemical composition to their own tissues; substances to be used as fuel in producing the activities of their bodies. In short, they require as food the very organic substances which plants spend their lives in making: viz., starches, fats and albuminous bodies. These substances must be supplied to animals ready made, so that directly or indirectly, through the medium of other animals, all these complex substances are the result of work done by vegetable life.

The chief acts of animal protoplasm are oxidations, a slow burning away of its substance, which results in the production of inorganic materials like those used by plants as food.

Plants use simple food stuffs, and from them manufacture complex combustible materials, and thus store up the energy of the sun's rays in their textures.

Animals, on the other hand, use complex food stuffs to renew their tissues, which are constantly being oxidized, and by this means the energy for the performance of their active functions is set free.

Although the various kinds of food stuffs used by animals are

highly organized and like the animal tissues in composition, yet they cannot be admitted at once into the economy without having undergone a special preparation, which takes place in the digestive tract, where the various food stuffs are so changed as to allow them to pass into the fluids of the body.

We shall first consider the food stuffs, next their preparation for absorption (digestion), and then the means by which they are distributed to the tissues (circulation). The final step in tracing the assimilation of the food is to follow the intimate processes which go on between the blood, which carries the nutriment, and the different tissues.

CLASSIFICATION OF FOOD STUFFS.

There are two portals, namely, the lungs and the alimentary canal, by which new materials normally enter the animal body. Within the lungs the blood comes into close relation with the air, and takes from it oxygen. The oxygen is then carried to the various tissues, where it aids in the combustion accompanying the life and functions of these tissues. Oxygen is the most abundant element in the body, taking part in almost every chemical change, and its continuous supply is more immediately necessary for life than that of any other substance, yet it is not counted as food, because tissue oxidation is distinguished from tissue nutrition.

The details of the union of oxygen with the blood will be found in the Chapter (XIX) on Respiration.

It is then only to the liquid and solid portions of the material income of an animal-that, in short, which it must busy itself to obtain that the term "food" is applied. These are introduced into the alimentary canal, where the nutrient materials are separated and prepared for absorption, while the portions which are . not useful for nutrition are carried away as excrement. We are, therefore, quite prepared to hear that the really nutritious food stuffs are composed of materials which are chemically like the tissues, although, as we shall see, we have no grounds for believing that the different chemical groups of nutritive stuffs are exclusively destined to replace corresponding substances in the