of the eye, 10, and also the posterior, 11, or that part behind the iris and in front of the crystalline lens, 12. The crystalline lens is a humour like a thick, transparent jelly, enclosed in a transparent bag, and attached to the choroid, so as to keep it in its place immediately behind the pupil. As a whole, it forms a double-convex lens (OPTICS, p. 28), but with the posterior surface more convex than the anterior. Filling the whole cavity behind the crystalline lens is the vitreous 1 humour, a glassy, transparent fluid, of the consistence of thin jelly. The optic nerve enters immediately behind the eye, 2 (fig. 61), and is spread out into the retina.

All the movements of the eye are performed by six muscles: four attached directly opposite to each other, above, below, and at each side, 9, 13, 10, and 11-12; and two oblique muscles, which turn the eyes

towards any of the corners. The act of thus pulling the eye forward is effected by these oblique muscles, 7, 8, passing through pulleys, as 6, in fig. 61. A very important appendage of the eye is the eyelid, which serves to protect the eye when closed, and also to sweep off any dust or impurity, by coming down over the eye, which it can do so rapidly as not to interfere with the sight. And in order to prevent a disagreeable rubbing in doing this, there is a fluid known as tears (when in more than an ordinary quantity), being continually poured on the upper part of the eye from the lachrymal2 gland. This fluid is carried off (except when so abundant as to overflow the eyes) by a little canal which communicates with the nose. The entrance to it may be seen on the inner edge of the under eyelid, almost in the inner corner.

Fig. 61.

When diverging (OPTICS, p. 26) rays of light, from any point, fall on the eye, those that fall on the opaque sclerotic, or white part of the eye, are lost. The cornea being transparent, the rays that fall on it pass on; then those that fall on the coloured iris are absorbed, while some pass through the pupil. After traversing the aqueous humour, which probably has little effect in changing their direction, the rays strike on the crystalline lens; the latter, on account of its convexity and comparative density, has strong refracting power, and after passing through it and the vitreous humour (the function of which is chiefly to support the retina), the rays are drawn to a point, or focus, on the retina. When rays from all the points of a body are thus thrown on the retina, the impression conveyed to the brain is a complete image of the object.

2 From Latin lachryma, a tear.

ZOOLOGY.

Introduction.

ZOOLOGY-from the Greek zōon, an animal, and logos, a discourse-is the branch of Natural History which treats of animals. In the largest sense of which the term is capable, it may be held to include man as part of its subject; but the term is not in general thus employed. Yet man has much in common with the lower animals; and a knowledge of human anatomy and physiology is necessary for the successful study of the structure of animals and the phenomena which life displays in them.

Of the three kingdoms of nature two are classed together as organic-the animal and the vegetable, the mineral being set apart as inorganic. Both plants and animals consist entirely of organs, and internal movements are kept up by fluids in cells and vessels, during the whole existence of the individual. It is not easy to define the term animal, familiar as is the idea conveyed by it. If we say that an animal is a creature having life, our definition is not perfect, for plants also have a kind of life, although we readily note a great difference between it and that which belongs to animals. When we study the lowest forms that nature presents to us in both kingdoms, it is sometimes difficult to determine to which kingdom a particular kind of creature belongs, although it is probable that our difficulty may be owing to our imperfect acquaintance with the subjects of our study, for no such difficulty is felt as to any of the higher kinds, the phenomena of whose life are better known to us. Perhaps the power of voluntary motion ought to be regarded as the true distinguishing characteristic of animals. When we think of animal life, we always associate with it the idea of mind-of a will, and feeling, or a capacity of pleasure and pain. To plants we do not ascribe will or feeling, even in the lowest degree. We associate no such idea even with the phenomena of irritability in the sensitive plants, or in flowers expanding under the sunshine, and turning toward the sun. If there be another distinction universally subsisting between animals and plants, it is that animals feed only on vegetable or animal substances; whereas the whole nutriment

of plants consists of inorganic substances, decomposition necessarily taking place before any organic substance can afford food for a plant.

Other peculiar characteristics of animals readily present themselves to the mind, but are not universal like these. Thus, animals are generally endowed with the power of locomotion, but some are destitute of it during the greater part of their existence. The animals with which we are most familiar all possess the senses of sight, hearing, &c.; but there are multitudes of creatures low in the scale of animal life which are destitute of these, having no brain nor manifest nervous system, and no organs of special sense, although it seems that they are generally sensible of the contact of external objects, and they manifest an avidity for food. Animals generally have a mouth and a stomach-indeed, some of the lower kinds may be described as consisting of little more, and very much resembling a bag, the mouth of which is capable of being opened and shut

-but there are some which have neither mouth nor stomach, properly so called, and imbibe their nourishment through their skin at all parts of their body. The animalcules of the genus Amaba1 or Proteus 2 resemble small masses of jelly, which seem to flow rather than crawl or glide over objects. For locomotion they protrude a portion of the substance of the body in any direction, the remainder following it. When one of them finds a particle of matter suitable for food, it envelops it, the body opening at any point to receive it, closing round it again, and retaining it till all that is capable of assimilation is extracted from it, and absorbed into the living jelly.

Fig. 62.-Amoeba radiosa.
a, young Amœba; b, another specimen.

In all probability the largest animals that exist are certain species of whale; but we are acquainted with many animals so small, that their existence can only be discovered by the aid of a powerful microscope, myriads inhabiting a single drop of water, whilst there may be others still more minute, which no optical instrument yet invented enables us to discern. Take any drop of water from the stagnant pools around us,' says Professor Rymer Jones, from our rivers, from our lakes, or from the vast ocean itself, and place it under the microscope; you will find therein countless living beings moving in all directions with considerable

1 Greek, vicissitude.'

2 From Proteus, a being of the Homeric mythology, who sought to elude observation by continually changing his shape.

swiftness, apparently gifted with sagacity, for they readily elude each other in the active dance they keep up. Increase the power of your glasses, and you will soon perceive, inhabiting the same drop, other animals, compared to which the former were elephantine in their dimensions, equally vivacious and equally gifted. Exhaust the art of the optician, strain your eyes to the utmost, until the aching sense refuses to perceive the little quivering movement that reveals the presence of life, and you will find that you have not exhausted nature.'

Classification of Animals.

The number of different kinds of animals is very great, vastly exceeding that of plants; and the diversities amongst them are very wide, not merely in size, but in structure and habits. Classification, therefore, is absolutely necessary, in order to obtain an intelligent view of the animal kingdom; but there are some groups which at once present themselves to the observer as distinctly marked, and which therefore have been universally recognised from the earliest times. Such is the great group of Birds, or that of Fishes; although observation more exact was necessary to exclude bats from the former, and whales from the latter. The greater group of Vertebrate Animals, in which both Birds and Fishes are included, is also so natural that it must have a distinct place in every system of classification. We cannot here give an account of the various systems of classification which have been proposed. That of Cuvier has been generally received since the beginning of the present century. Cuvier regarded the animal kingdom as consisting of four great divisions -Vertebrata, Mollusca, Articulata, and Radiata. The first division, Vertebrata or Vertebrate Animals, consists of animals having an internal bony skeleton, the principal part of which is a back-bone or vertebral column, composed of short joints, to which all the other parts are attached. All the highest classes of animals belong to this division-Mammals, Birds, Reptiles, and Fishes. Man himself, considered as to his corporeal frame, belongs to it. All the other divisions of the animal kingdom are united under the common designation of Invertebrata or Invertebrate Animals; but they differ far more widely from each other than the classes of vertebrate animals, and are therefore regarded by Cuvier as forming three great groups, each equal in rank with the Vertebrata. His second division, Mollusca,1 consists of animals which have no skeleton, external or internal. Their bodies are always soft and covered with a skin, and often protected by an external shell, which they secrete. Snails, slugs, oysters, mussels, cockles, and most of the other creatures popularly called shell-fish, belong to this division. Cuvier's third division, Articulata 2 or Articulated

1 From Latin molluscus, soft.

2 From Latin articulatus, jointed.

Animals, contains animals which, however great their other differences, have this character in common, that they are composed of segments articulated or jointed together in a line. Insects; Crustaceans, as crabs and lobsters; Arachnids, as spiders and mites; and Annelids, as leeches and worms, belong to this division. These three great divisions continue to be recognised as Cuvier established them. Not so, however, his last great primary division, Radiata1 or Radiated Animals, which takes its name from the arrangement of the organs of sense and motion in rays around a centre. Cuvier placed in this division many groups of animals in which no such arrangement is to be discerned, and which differ very widely from the true Radiata. With regard to these-the lowest tribes of animals-the progress of discovery has recently been very great, consequently a modification of Cuvier's system has become necessary, and new groups have been constituted.

Protozoa.

The very lowest tribes of animals, those of most simple organisation, are now generally regarded as a distinct primary division of the animal kingdom, to which the name Protozoa 2 is given. With the exception of sponges, almost all the Protozoa are minute, and most of them are microscopic. Except a few that are found in the bodies of other animals, they all live in water. In general, their bodies consist simply of a mass of gelatinous matter. They have no nervous system, and therefore no organs of special sense; and none of them, except one group, have a mouth or an intestinal canal.

Rhizopoda. Among the lowest of the Protozoa, those most remarkable for the simplicity of their organisation, or want of distinct organs, are the Rhizopoda,3 a class particularly important on account of their universal diffusion, the multitudes in which they exist, and the great variety of kinds. The Amoeba, already mentioned, may be regarded as a type of this class. All of them have, like it, the power of throwing out in any direction processes of the gelatinous substance of which they are composed, which are sometimes broad and lobe-like, but in other species are slender and of great length. The most interesting of the Rhizopoda are those which constitute the group called Foraminifera, which are covered with a shell, and to which we owe the countless multitudes of minute fossilshells forming great part of chalk and some other rocks, whilst the shells of still existing species abound in the sand of our sea-shores. These creatures are merely little particles of a kind of jelly; yet the shells which they produce are of the most exquisite beauty, and exhibit a

1 From Latin radius, a ray.

4

2 From Greek prōtos, first, and zōon, an animal.

3 From Greek rhizon, a root, and pous, podos, a foot.
4 From Latin foramen, a pore, and fero, to bear.