comprised within the great Crustacean family. The chief division is suggested by the fact that some have jaws for masticating food, while others have merely a kind of beak, or tubular apparatus, through which they draw their food by suction; and there is yet a third class, whose mouth is surrounded by legs, the bases of which are used as jaws. All Crustacea are therefore divided into the Maxillosa, or masticating; the Edentata, or Hausstellata, the suctorial; and the Xiphosura, or the sword-tailed. The masticating and the suctorial, again, are each divided into various great sections, with further subdivisions into orders and species.

The most interesting feature of the masticating Crustacea is the jaw-foot, represented in the following cut. In the sucking Crustacea, which are

Fig 1

Fig 3

Fig4

parasites, and feed on other animals, we have, instead of the jaw-feet, a tube, or proboscis, containing within a pair of animal-lancets. These make the wound through which the juices are afterwards drawn. The food, thus prepared, passes Tig 2 through a short tongueless mouth into the œsophagus, thence into the stomach, where we find an extraordinary apparatus for tearing and grinding the food, consisting of tubercles, or teeth. The liver is largely developed in many Crustacea, especially in the decapods, or ten-footed, as every epicure in shell-fish well knows, for in this order are included lobsters, crabs, and shrimps. The blood is either colourless, or of a slightly bluish tinge, and is circulated, it is believed, in a similar manner to that of the molluscs. The heart is single, and of various forms—square, cylindrical, &c. The appearances presented by the heart in some Crustacea have been likened to the effect produced by the superposition of a number of stars, the rays of which do not correspond. The respiratory process in the Crustacea is a very interesting and very complex subject. Generally, breathing takes place through the branchia, or gills-an organ familiar to all eaters of the crab, in the form of a number of leaf-like processes, arranged in two groups, the points coming nearly together. In some Crustacea no special respiratory apparatus can be discovered, and oxygen is then supposed to be drawn directly through the external integument. In others, the gills float in the water, like so many feathery tufts. The Phyllopoda, or gill-footed, are distinguished by an extension of their legs, in order to make the latter subservient to respiration -a fact which explains the movement sometimes seen in the feet of such animals, when all the rest of the body is quiescent. The land-crabs possess

Fig 5

Fig. 59.—JAW-FEET OF THELPHUSA

FLUVIATILIS.*

* 1. Right external jaw-foot; A, its internal blade; a, b, c, d, e, f, its various articulations; B, its external blade, or palp. 2. Jaw of the third pair, with its palp. 3. Mandible, with its palp. 4. Upper lip. 5. Lower lip, sometimes called the tongue.

special contrivances for the moisture requisite to enable the gills to perform their functions. For the same reason these animals never go far away from damp situations. With them the activity of the breathing organs is so great that they cannot draw the requisite supply of oxygen from water, and consequently they die, if long immersed. We may fitly

Fig. 60.*

illustrate the foregoing remarks by the annexed engraving, showing the circulatory and respiratory system of the lobster.

There is no brain, strictly speaking, in Crustacea, but there is a tendency towards its formation visible in the centralization of the nervous functions in the anterior part of the ganglionic chain. The place of the brain in the higher animals is occupied, in Crustacea, by the ganglions, which possess individually the faculty of receiving sensations and determining motion, and which act through the nervous cords.

The eyes of Crustacea exhibit a great variety of structure. In certain classes we find smooth, or simple eyes, two or three in number, formed by a mere modification of the tegumentary membrane, with a mass of gelatine behind it, acting as the vitreous humour, and which is in connection with the optic nerve. The king-crab is an example. These simple eyes, or stemmata, are always immovable, and sessile, or sitting. Other classes have what are called intermediate eyes, in which the cornea is still undivided externally, but has behind a number of simple eyes, or lenses, each with its own vitreous humour, and its own connection with the optic nerve. Lastly, (and this refers to the great majority of Crustacea,) we have compound eyes, presenting a number of facets, each with its own ocular compartment behind. These facets are square in the common craw-fish, hexagonal in the crab, and so on. There is a remarkable fossil species, in which there are four hundred of these facets, the whole so exquisitely arranged for mutual aid, that where the range of one ceases, that of the next begins. There are generally two of these compound eyes, although sometimes they are so closely united as to appear but as one. Sometimes the compound eyes are movable, and sometimes they are supported on a pedicle, or stalk, moved by special muscles.

*h, Heart; s, sinus, or dilated vein, receiving the blood which comes from different parts of the body, and is thence sent to the branchia, b, from whence it returns to the heart by the branchial veins, v.

[END OF ZOOLOGY.]

PART IV.

THE

PHYSICAL HISTORY OF MANKIND.

THE PHYSICAL HISTORY OF MANKIND.

CHAPTER I.

INTRODUCTORY.-ON THE OBJECTS AND INTEREST OF THE STUDY OF MAN.

As a motto to this division of our "Home Tutor we might most appropriately adopt the line of the poet

"The proper study of mankind is man."

We do not, however, wish it to be inferred that our own subject is above all others that which ought peculiarly to interest mankind. In fact, the expression of the poet is rather an assertion of what is, than what ought to be. There are many reasons why man should be, to himself, the most interesting and absorbing of all studies. Surrounded by an external creation constantly suggestive of thoughts of the beautiful, the grand, and the sublime, man yet turns his mind upon himself, and finds that it is in his own wonderful nature that he must look for the true source of the beauty and the wonder of the external world. The heavens and the earth are alike unfolded to the eye of lower beings; but they kindle no emotions in their consciousness, and add no happiness through thought to their being.

The thought and consciousness of man, his soul and understanding, have ever been to him a theme of intensest interest, and must, whilst the human race continues on the earth, occupy a first place in his regards. Setting aside the thought by which man is enabled to reason and study as a part of his nature, his relation to the rest of creation must give him a deep interest in his race. When we regard the external world with attention, we see clearly that the distinction between the material world and that of organic existences is very great. However wonderful may be the movements of the heavenly bodies, and the grand features which mark the physical geography of the surface of the earth, we instinctively feel that there are higher agencies and a more mysterious power at work in the growth and functions of the simplest plant of the field. We speak of rivers, seas, rocks, and mountains as existing, but of plants as living. We feel that there is more proof of Wisdom and Goodness in the creation of a plant than of a stone. We then pass on from plants to animals, and we observe in passing up from class to class, till we arrive at the highest, the gradual complication of their structure, the increase of their functions, the numerous new relations they sustain to the mineral and vegetable world, and we feel that animals are more wonderful structures than plants, and they accordingly take a stronger hold of our sympathies, and demand a larger part in our affections. If then we carry on our view to man, and find him embracing all that is wonderful in the creations below him, and exhibiting new and unlooked-for adaptations to his position in the world, we shall see how it is that man, as an object external to himself, becomes one of the most interesting and absorbing subjects of his own study.