allied races with which insects have any chance of being confounded. To render this the more obvious, a brief notice may be taken of a few of the more prominent peculiarities presented by each of the other articulated classes, when compared with that in question. The Myriapodes make by far the nearest approach to them in essential properties, the internal structure being almost identical, while many of the external parts are similar: thus there are generally two composite eyes, two antennæ, and oral organs similar to those of masticating insects. The differences, however, are sufficiently striking, and consist of the numerous segments, without any division of the body into thorax and abdomen; in the number of feet, always exceeding six, and sometimes amounting to two hundred; and in the body acquiring with age an increase in the number of the component segments. The Arachnides generally have the head soldered to the thorax, and many of them seem to have no other incisure than that which separates the thorax from the abdomen; no antennæ nor composite eyes; more than six feet, and the generative organs placed, with very few exceptions, under the belly before the middle. In that section of them named Pulmonaria, after the air has been admitted by stigmata, it is received by a kind of sacs, analogous to the lungs of vertebrate animals, and the circulation in consequence is pretty complete; in the other division, Trachiana, the respiratory organs resemble those of insects, and the circulation is therefore less perfect. The Crustacea, agreeing in very many points with
