The diagram on the opposite page roughly represents (for the relations of animals can not be expressed on a plane surface) the relative positions of the branches and classes according to affinity and rank.*

SERIES I. — PROTOZOA

Animals whose bodies consist of a single cell, the process of reproduction being by division or by budding, but never by means of true eggs.

In structure the Protozoa are the simplest of animals, consisting of only a single cell. They are microscopic in size and aquatic in habit, though in certain stages of their lives (encystation) many of them may endure dryness for weeks or months. Their bodies consist mainly or wholly of protoplasm, which may or may not be covered by a cuticle or by a shell-like excretion of lime, chitin, or flint, or inclose spicules of the latter substance. The various individuals may live separately as single, independent organisms, or they may be organically joined together in clusters called colonies. They exhibit all the essential functions of life — nutrition, growth, nervous properties, and reproduction. They feed upon minute algæ, bacteria, vegetable dé-· bris, and upon other microscopic animals. Some forms are parasitic. It has been shown by experiment that many species are sensitive to changes in the amount of illumination to which their bodies are exposed and to various colors of light; that they are attracted or re

*The student should master the distinctions between the great groups, or classes, before proceeding to a minuter classification. "The essential matter, in the first place," says Huxley, "is to be quite clear about the different classes, and to have a distinct knowledge of all the sharply definable modifications of animal structure which are discernible in the Animal Kingdom."

pelled by the presence of different chemical substances in the water; and that they are sensitive to contact with foreign bodies and with one another. Thus it is proved that these simple organisms possess the rudiments of the nervous properties seen in the higher animals. Certain species contain a green coloring substance (hæmatochrome) which is chemically allied to the chlorophyll of plants. Others, again, pass through amoeboid stages resembling similar phases in the development of some of the lowest plants. Because of these resemblances some of the Protozoa are almost indistinguishable from the lowest members of the plant kingdom (Protophyta).

On account of the apparent simplicity of their structure, it is difficult to select features by means of which the animals in this group may be classified. The difficulty is further increased by the fact that in the course of their development some forms pass through stages in which they resemble other species in the same branch. In every case, however, it is found that certain phases of their development predominate, and these wellmarked phases permit of dividing the Protozoa into five classes.

These are Protozoa which are predominantly amoboid in shape and which move by means of pseudopodia, as the slow-moving protrusions of the protoplasmic body substance are called (Figs. 1, 213). The body usually contains a nucleus and a contractile vacuole. The common amœba or proteus animalcule belongs in this class (Fig. 1). Some of the Rhizopods secrete shells of chitin (Arcella), or construct a covering made of particles of sand (Difflugia). Both of these organisms are found in fresh water in America. The most primitive representative of the group is Protamaba, in which

neither nucleus nor contractile vacuole has been discovered. Pelomyxa, a fresh-water form, may reach the size of eight millimeters (.3 of an inch) in diameter.

An amoeba is a naked fresh-water Rhizopod, containing a nucleus and a contractile vacuole, the body sub

FIG. 1. Amaba, showing the structure of the body and the changes which take place during division. The dark body in each figure is the nucleus; the transparent circle, the contractile vacuole; the protrusions of the body substance, pseudopodia; the outer, clear portion of the body, the ectosarc; the granular portion, the endosarc; the granular masses, food vacuoles. Much magnified.

stance consisting of two rather distinct layers, the outer being quite clear and transparent, while the inner is usually filled with granules and ingested particles. During movement the shape of the body is constantly changing, owing to the protrusion and withdrawal of the pseudo

podia. Food is taken into the body at any point, there being no mouth.

A Foraminifer differs from an amoeba in having an apparently simpler body, the protoplasm being without layers or cavity; its pseudopodia are long and threadlike, and may unite where they touch each other. It has the

property of secreting an envelope, usually of carbonate of lime. The shell thus formed is sometimes of extraordinary complexity and singular beauty. In addition to the terminal aperture, it is generally perforated by

FIG. 2.

Rhizopods: a, shell of a monothalamous, or single-chambered, Foraminifer (Lagena striata); b, shell of a polythalamous, or many-chambered, Foraminifer (Polystomella crispa), with pseudopodia extended; c, shell of a Radiolarian, one of the Polycystines (Podocyrtis schomburgkii).

innumerable minute orifices (foramina) through which the animal protrudes its myriad of glairy, threadlike arms. The majority are compound, resembling chambered cells, formed by a process of budding, the new cells being added so as to make a straight series, a spiral, or a flat coil. As a rule, the many-chambered species have calcareous, perforated shells; and the onechambered have an imperforated membranous, porcelaneous, or arenaceous envelope. The former are marine. There are few parts of the ocean where these microscopic shells do not occur, and in astounding numbers.