appendages for flight, indicating the metamorphosis to which the animal is subject when young; legs most commonly reduced to six. Arachnida: Distinguished from Crustacea by having their respiratory organs always internal, opening on the sides of the abdomen or thorax to receive the respirable fluid. Sometimes these organs perform the office of lungs, and then the circulation takes place by means of a dorsal vessel, which sends forth arterial, and receives venose branches. Sometimes they are tracheæ or air-vessels, which, as in the class Insecta, replace those of circulation. These have only the vestige of a heart, or a dorsal vessel alternately contracting and sending forth no branch. The absence of antennæ, the reunion of the head with the thorax, a simple trachea but ramified and almost radiating, serve to distinguish these last Arachnida, or the most imperfect of insects, which respire only by tracheæ[28]." Under this head he observes—"Of all these characters, the most easy to seize and the most certain would doubtless be, if there were no mistake in it, that of the absence of antennæ; but later and comparative researches, confirmed by analogy, have convinced me, that these organs, under particular modifications it is true, and which have misled the attention of naturalists, do exist[29]:" and he supposes, from the situation and direction of the mandibles of the Arachnida, corresponding with that of the intermediate pair of antennæ in Crustacea, that they really represent the latter organs. If this supposition be admitted, their use is wholly changed; the palpi, in fact, executing the functions of antennæ, which probably induced Treviranus to call them Fühlhörner (Feelinghorns). Perhaps these last may be regarded as in some sort representing the external antennæ of the Crustacea? With regard to Insecta, their antennæ seem to disappear in the Pupiparæ Latr., or the genus Hippobosca L.

The above definitions of the Arachnida by these two celebrated authors, appear to me the reverse of satisfactory. When we are told of animals included in it, that some breathe by gills and others by tracheæ, that some have a heart and circulation and others not, we are immediately struck by the incongruity, and are led to suspect that animals differing so widely in the fountains of life ought not to be associated in the same class. A learned zoologist of our own country, Dr. Leach, seems to have made a nearer approach to a classification in accordance with the internal organization, by excluding from Arachnida the Acari and Myriapoda.

Sub-kingdom Annulata Cuv.

Mr. MacLeay, on whose system I shall now say a few words, divides his sub-kingdom Annulosa into five classes, namely, Crustacea, Ametabola, Mandibulata, Haustellata, Arachnida. From the Crustacea he goes by the genus Porcellio Latr. to Iulus[31], which begins his Ametabola: these he connects with the Mandibulata, by Nirmus, which he thinks approaches some of the corticarious Coleoptera[32]. This class he appears to leave by the Trichoptera Kirby, and so enters his Haustellata by the Lepidoptera[33], and leaves it again by the Diptera by means of the Pupiparæ Latr., especially Nycteribia, connecting this class with the Arachnida, which he enters by the Hexapod Acari L.[34], and these last he appears to leave by the Araneidæ, and to enter the Crustacea by the Decapods[35]: thus making good his circle of classes, or a series of Annulose animals returning into itself. Mr. MacLeay's whole system upon paper appears very harmonious and consistent, and bears a most seducing aspect of verisimilitude; but it has not yet been so thoroughly weighed, discussed, and sifted, as to justify our adopting it in toto at present: should it, however, upon an impartial and thorough investigation, come forth from the furnace as gold, and be found to correspond with the actual state of things in nature, my objections, which rest only upon some parts of his arrangement of Annulosa, would soon vanish. Some of those objections I will state here, and some will come in better when I treat of the Systems of Entomology. My first objection is, that his Ametabola, Mandibulata, and Haustellata, approach much nearer to each other than they do to the other two classes of his circle, or than even these last to each other; so that under this view it should primarily consist of three greater groups, resolvable, it may be, into five smaller ones. My next objection is, that he has also considered the Trachean and Pulmonary Arachnida as forming one class. Whether an animal breathes by gills or tracheæ, or has a circulation or not,