2204. Where the branchiæ have assumed the laminated form, they are surrounded by a similarly formed covering or operculum. The shells of the Mollusca are, as regards their signification, none other than branchial opercula, or gill-covers; the same holds good also of the gasteropodal and crustacean shells, and at bottom of every calcareous and horny covering of the body. 2205. To the same category belong also the opercula of Fishes, and even their scales. Fundamentally, the whole epidermis is only a product of respiration or oxydation. 2206. The first saccular or cæcal inversion of the integument, as in the Leeches, Molluscs, and Snails, is at once a predominance of the tegumentary, obtained through the medium of the branchial formation, whereby the skin commences to become a self-substantial organ of respiration. 2207. In the Scorpions the branchiæ are introverted sacs or cysts, into which, however, instead of water, air already enters. 2208. This insaccation is converted in the Spiders into more distinct air-cysts, which finally ramify in the higher insects and become true tracheæ. 2209. Lastly, the respiratory system obtains the upper hand to such an extent, that, together with internal air-tubes, external branchial laminæ are also developed, as in the Molluscs; but in them the tracheæ obtain the preponderance over the blood-vessels, so that these laminæ dry up and become wings or fins. 2210. The wings of Insects are branchial laminæ, converted into air-organs. 2211. The wing-coverings or elytra are branchial opercula, and correspond to the shells of bivalve Mollusca. 2212. Every insect therefore must properly possess four wings and two wing-coverings, of which last, however, rudiments only appear to be left in the nocturnal Lepidoptera. 2213. In the higher organized animals those gills only which are nearer the head are persistent, the posterior or lateral branchiæ being gradually arrested. 2214. These lateral branchiæ remain in Fishes as lateral mucous openings, which constitute the lateral line. 2215. The cervical branchiæ are limited to the number five, which has already begun to be established in the Crustacea; namely, at the origins or roots of the five anterior pairs of feet. 2216. The number five probably derives its origin from the vegetable kingdom, and that indeed from the genesis of the pinnate leaves, so that one kind of numerical law appears to prevail with respect to this organ in both organic kingdoms. It is probably also an imitation of the five organs of sense. 2217. The vessels of the branchiæ of Fishes are accompanied by bony rings, which correspond to the feet of Crustacea. 2218. All Fishes have, with few exceptions, five branchial arches. 2219. When in Fishes the sarcous system begins to produce viscera, then the five branchial foramina pass inwards, and only a single respiratory aperture is left for them in the fleshy body; namely, the external branchial foramen.