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599. The division of this Sub-kingdom into Classes, is principally founded on the organs of locomotion; which, as we have seen, are so characteristic of it. We may first form two groups, distinguished by the presence, or absence, of distinctly-articulated members. In the former, the locomotive power is for the most part consigned to these appendages; and the trunk is usually encased in a hard envelope, in which, by the union of segments that were originally distinct, we sometimes almost lose the traces of the characteristic division. This is especially the case with the Crab and its allies; in which the different parts of the body are quite immoveable upon each other. On the other hand, in the lower group, in which distinct members are wanting, the locomotion of the animal is chiefly effected by the movement of the body itself; and this is permitted to the utmost extent, by the softness of the integument, in which the intervals of the articulations are scarcely distinct from the rings themselves, as in the Leech or Earth-worm,-so that here, too, the division into segments becomes indistinct, from the opposite cause. It is in the Centipede (Fig. 292) that we have the best example of the division of the body into segments, which is characteristic of the entire group; together with an articulated structure in the limbs. In the classes of Insects, Crustacea, Spiders, &c., the equality of the segments disappears; whilst in the Leech and Worm tribes, the members disappear.

600. The higher division of the Articulated series may be arranged into the following classes :

:

I. INSECTS; characterised by the division of the body into three distinct portions, the head, thorax, and abdomen; by the possession of antennæ on the head, of three pairs of legs, and (in general) of one or two pairs of wings; and by their aerial respiration.

FIG. 292. CENTIPEDE.

II. ARACHNIDA, including the Spiders, Scorpions, and Mites;

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CLASSIFICATION OF ARTICULATA.

characterised by the division of the body into two distinct portions, the cephalo-thorax (made up of the head united to the thorax) and abdomen; by the possession of four pairs of legs; by their want of antennæ; and by their aerial respiration.

III. CRUSTACEA, or Crabs, Lobsters, &c.; distinguished by their aquatic respiration; and by the possession of from five to seven pairs of legs; the body sometimes divided nearly as in Insects, sometimes even more concentrated than in the Arachnida, and sometimes formed on the plan of that of the Myriapoda.

IV. MYRIAPODA, the Centipede tribe; characterised by the want of distinction between thorax and abdomen; by the equality of the segments of the body,-the head, however, being very distinct; by the large number of legs, of which there are seldom less than twenty-four pairs; and by their aerial respiration.

Intermediate in some respects between the two divisions of this Sub-kingdom, we may rank the following remarkable group :

V. CIRRHOPODA, or the Barnacle tribe; in these, there are no locomotive members in the adult, although the young possesses them; there are, however, a series of jointed tendril-like appendages, which probably serve both for respiration and for the acquirement of food; and the animals remain attached to one spot, during all but the early period of their lives; their respiration is entirely aquatic.

601. In the Second division of Articulata, characterised by the absence of articulated members, we meet with but three classes.

VI. ANNELIDA, the Leech and Worm tribe; characterised by the extension of the body into numerous segments, which present scarcely any differences from each other; by the presence of a distinct circulating system, and of respiratory organs; and by the possession of a well-developed nervous system.

VII. ENTOZOA, or Intestinal Worms, in which the Articulated form is presented in (as it were) a still more degraded state; the segments being yet more completely repetitions of one another, and often capable of existing separately; special organs of circulation and respiration being for the most part wanting;

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and the nervous system being very indistinct,-never presenting the double ventral cord in a well-developed condition: these have very frequently no distinct head.

VIII. ROTIFERA, or Wheel-Animalcules, a group of minute animals, in which the Articulated structure is often very indistinct, and which are especially characterised by the possession of cilia, or little hair-like filaments, arranged in rows in the neighbourhood of the mouth, and serving by their movements, both for the acquirement of food, and for the aeration of the fluids of the body.

CHAPTER VIII.

OF THE CLASS OF INSECTS.

602. THE class of Insects is pre-eminent, not only amongst the divisions of the Articulated series, but in the whole Animal kingdom, in regard to the number of distinct species which it includes; and it is probably unsurpassed by any, save the Infusory Animalcules, in regard to the number of individuals at any time existing on the earth's surface, which belong to the numerous and diversified races comprehended in it. The whole sub-kingdom of Vertebrated animals may probably be estimated as not containing above 30,000 species, a number which is surpassed by the Beetle tribe alone; and we should be probably not far wrong in saying, that the number of species of the whole class, already known, exceeds that of all other animals put together. Moreover, on account of the small size of all, and the very minute size of a large proportion, of the animals belonging to this class, it is probable that the number of species already known is far surpassed by that which remains to be discovered. Even in our own country, new species of Insects are continually being discovered, by the industry of observers who devote themselves to this particular branch; and the number at present unknown must of course be far greater, in countries that have been less completely explored. Of the importance of this class in the economy of Nature, several striking proofs will be given

hereafter.

603. The tegumentary skeleton of Insects, that is to say, the hard skin of these animals, sometimes preserves a certain degree of flexibility; but generally presents a consistency analogous to that of horn. It must not be thought, however, that its tissue is really of that nature; for Chemistry teaches us that it is composed of very different materials, and that a particular

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substance named Chitine, forms the base of it; but there is reason to believe, that it possesses a definite organic structure; and that it consists of one or more layers of cells, in the cavities of which this substance is deposited. We see in it a great number of pieces, which are sometimes soldered (as it were) together; whilst in other instances they are united by soft portions of the skin, and thus possess a greater or less freedom of motion.

604. The body of the Insect, as we have already said, is divided into a certain number of rings, placed end to end; and

FIG. 293.-ANATOMY OF THE EXTERNAL SKELETON OF AN INSECT.

in this series of segments, we distinguish three portions, to which are given the names of Head, Thorax, and Abdomen. The members or appendages which spring from the several segments, have a structure analogous to that of the trunk of the animal: being composed of tubes or of hollow plates, placed end to end, and enclosing in their interior the muscles and nerves destined to put them in motion.