It is to be observed with regard to the Arachnida of the French school, that the class as laid down by them includes several animals that have no circulation, and breathe by tracheæ, of which description are the mites (Acarus L.), and the harvest-men (Phalangium L.) &c.; and therefore it has been divided into two orders, Pulmonaria and Tracheana; but if the definition from the internal organization be adhered to, the latter should either remain with the class Insecta, or form a new one by themselves. Yet the animals that compose the Trachean order of Arachnida, their external form considered, are certainly much more nearly related to the spiders and scorpions than to any members of the class Insecta at present known. This circumstance, perhaps, may seem to throw some doubt upon the modern system of classification.

I must further observe, that the assertion of Treviranus, which appears to intimate that the respiration of the pulmonary Arachnida is the same with that of the Crustacea, is not quite correct, since in the latter the branchiæ or gills are external, and in the former internal, the air entering by spiracles before it acts upon them[15].

It may not be amiss in this place to lay before you the principal points in which the Crustacea and Arachnida agree with Insecta, and also those in which they differ.

The Crustacea agree with Insecta in having a body divided into segments, furnished with jointed legs, compound eyes, and antennæ. Their nervous system also is not materially different, and they are both oviparous. They differ from them in having the greater insections of the body less strongly marked; in the greater number of legs on the trunk, the anterior ones performing the office of maxillæ: in their eyes usually on a moveable footstalk: their palpigerous mandibles; and their four antennæ at least in the great majority. But the principal difference consists in the internal organization and the fountains of vitality; for the Crustacea have a double circulation, the fountain of which is a heart in the middle of their thorax[16]. They have too a kind of gizzard and liver, at least the Decapods[17], and their respiration is by gills. Genuine insects terminate their existence after they have laid their eggs[18]; but the Crustacea live longer, and lay more than once.

The Arachnida will be found to differ from insects more widely than even the Crustacea. They agree in their jointed legs and palpi; immoveable eyes; and in being covered with a coriaceous or corneous integument: but they differ in having a system of circulation; gills instead of tracheæ; their organs of generation double; and the females lay more than once in their lives. Their head also is not distinct from the trunk as in insects; they have no compound eyes; and their antennæ, if we admit the opinion on this head of MM. Latreille and Treviranus, that they have representatives of these organs, differ totally in structure, situation, and use, from those of the great body of insects. In the Araneidæ or Spiders, their body seems to have no segments or incisure but that which separates the abdomen from the trunk; and in the Scorpionidæ they are observable only in the abdomen. Other particulars might be enumerated in which these two classes differ from insects; but these will be sufficient to convince you that Aristotle and MM. Cuvier and Lamarck were justified in separating them.

The two last-mentioned authors made further improvements in Zoology. The latter, from the consideration of the general structure of animals, perceiving that Aristotle's Enaima were distinguished from his Anaima, by being built as it were upon a vertebral column, very judiciously changed the denomination, which was indeed improper, of "The Philosopher's" two sub-kingdoms, into that of Vertebrata or animals that have a vertebral column, and Invertebrata or those that have no vertebral column. These he distributes into three primary divisions according to their supposed degrees of intelligence—Thus:

* Apathetic Animals.1. Infusoria.
2. Polypi.
3. Radiata.
4. Vermes.
** Sensitive Animals.(Epizoaria.)
5. Insecta.
6. Arachnida.
7. Crustacea.
8. Annelida.
9. Cirrhipeda.
10. Mollusca.
*** Intelligent Animals.11. Pisces.
12. Reptilia.
13. Aves.
14. Mammalia.[19]

Profiting by the light afforded by the Aristotelian system, this eminent zoologist improved, we see, upon that of Linné, by resolving his Insecta into three classes, and his Vermes into seven, interposing the Linnean Insecta between the four first and three last, in which he was not so happy, since as to sense insects should certainly occupy the place he has here assigned to the Mollusca.

In the work from which I have taken this statement of Lamarck's system, that acute writer has given a sketch of another method of arrangement, in which he has made the first deviation from the beaten track of an unbroken and unbranching series. In the Supplement to the first volume, he has distributed the Invertebrata in a double subramose series—one consisting of articulate, and the other of inarticulate animals[20].

Upon Lamarck's system, most of the modern ones, with some variation, are founded. There is one, however, by a learned countryman of ours, that is more unique, sui generis, and I may add profound, than any that has yet appeared. I am speaking of that, you will perceive, of which our friend Mr. Wm. MacLeay has given a detailed statement in his Horæ Entomologicæ. In this he goes even far beyond what Lamarck has attempted in the above sketch, and substantiates his claim to be considered as one of those original thinkers, rari nantes in gurgite vasto, that do not appear every day. The following are the principal bases of his system.