provinces, in three of which (with the exception of the Crustacea and Arachnida belonging to his Annulosa) no circulation of blood is visible, but which obtains in the rest. These he names—

1. Acrita, consisting of the Infusory Animals, the Polypi, the Corallines, the Tæniæ, and the least organized of the Intestinal Worms.

2. Radiata, including the Jelly-fish, Star-fish, Echini, and some others.

3. Annulosa, consisting of Insecta, Arachnida, and Crustacea.

4. Vertebrata, consisting of Beasts, Birds, Reptiles, Amphibia, and Fishes.

5. Mollusca, including the numerous tribes of shell-fish, land-shells, slugs, &c., which, from their mucous or gelatinous substance, from their nervous system and the imperfection of their senses, return again to the Acrita, though connected with the Vertebrata by having a heart and circulation.

His next set of circles shows the sub-division of these five sub-kingdoms into classes—Thus:

In this scheme the osculant classes are those placed between the circles. In the Mollusca circle two classes are still wanting to complete the quinary arrangement of that sub-kingdom. I am not sufficiently conversant with the details of the animal kingdom at large to hazard any decided opinion upon Mr. MacLeay's whole system, or to ascertain whether all these classes are sufficiently distinct[21]. My sentiments with regard to those of the Annulosa I shall state to you hereafter.

Upon a future occasion I shall consider more at large the station to which insects seem entitled in a system of invertebrate animals, which will not accord exactly with that assigned by MM. Cuvier and Lamarck. But I am now in a field in which I have no intention to expatiate further, than as it is connected with the subject of the present letter. I shall therefore confine myself in what I have more to say to the definitions of Insecta that have been given by modern authors, beginning with that of the zoologist last mentioned. Insects form a part of his second group, which he terms sensitive animals (animaux sensibles), which group he thus defines: "They are sentient, but obtain from their sensations only perceptions of objects—a kind of simple ideas which they cannot combine to obtain complex ones. Charact. No vertebral column; a brain, and most commonly an elongated medullary mass; some distinct senses; the organs of movement attached under the skin: form symmetrical, by parts, in pairs[22]." This division of animals, from the kind and degree of sense and intelligence that they possess, seems rather fanciful than founded in nature, since many insects show a greater portion of them than many vertebrate animals. Compare in this respect a bee with a tortoise[23]. Lamarck divides his group of animaux sensibles into two sections, namely, Articulated animals, exhibiting segments or articulations in all or some of their parts; and Inarticulatcd animals, exhibiting neither segments nor articulations in any of their parts. Insecta, Arachnida, and Crustacea, belong to the first of these sections, which he defines as "those whose body is divided into segments, and which are furnished with jointed legs bent at the articulations[24]." Insecta he defines—"Articulate animals, undergoing various metamorphoses, or acquiring new kinds of parts—having, in their perfect state, six feet, two antennæ, two compound eyes, and a corneous skin. The majority acquiring wings. Respiration by spiracles (stigmates), and two vascular opposite chords, divided by plexus, and constituting aeriferous tracheæ, which extend every where. A small brain at the anterior extremity of a longitudinal knotty marrow, with nerves. No system of circulation, no conglomerate glands. Generation oviparous: two distinct sexes. A single sexual union in the whole course of life[25]." Arachnida he defines—"Oviparous animals, having at all times jointed legs, undergoing no metamorphosis, and never acquiring new kinds of parts. Respiration tracheal or branchial: the openings for the entrance of the air spiraculiform (stigmatiformes). A heart and circulation beginning in many. The majority couple often in the course of life[26]." I shall next add his definition of Crustacea: "Oviparous, articulated, apterous animals, with a crustaceous integument more or less solid, having jointed legs; eyes either pedunculate or sessile, and most commonly four antennæ, with a maxilliferous mouth seldom rostriform; maxillæ in many pairs placed one over the other; scarcely any under-lip; no spiraculiform openings for respiration; five or seven pair of legs; a longitudinal knotty marrow terminated anteriorly by a small brain. A heart and vessels for circulation. Respiration branchial with external branchiæ, sometimes hid under the sides of the shell of the thorax, or shut in prominent parts; sometimes uncovered, and in general adhering to particular legs or to the tail. Each sex usually double[27]."

I have given Lamarck's definitions of these three classes, all considered as Insecta by Linné, that by comparing them together you may be better enabled to appreciate the system of this author. On looking over the characters of the Arachnida as here given, you will see at once that it consists of heterogeneous animals—for in fact he includes in this class not only the Trachean Arachnida of Latreille, but the Ametabolia of Dr. Leach, or the Hexapod Aptera, and the Myriapoda.

I shall next copy for you Latreille's latest definition of Insecta and Arachnida.

"Insecta: A single dorsal vessel representing the heart: two trunks of tracheæ running the whole length of the body, and opening externally by numerous spiracles; two antennæ; very often upper