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.
2220. In the lower animals water or air passes in and out through the same respiratory aperture, but in Fishes these two courses are preserved distinct; for, except in the Lampreys, the water enters through the mouth, and passes out through the branchial aperture.
2221. Here the attempt is manifested in a still greater degree, to bring the process of respiration wholly under the control of what is animal, this being in the next place, and for the first time attained, when respiratory apertures only are left upon the head.
2222. The respiratory apertures of the head are the nostrils, which are suddenly manifested in Fishes, but are in them simply subservient to the sense of smell, and not as yet to the respiratory act.
2223. All the higher animals have, like fishes, branchial apertures in the neck, only they coalesce and become obliterated at an early period, or so soon as that respiratory process appears, when air passes through the nostrils. In the Salamanders and Frogs these branchial foramina are persistent for a longer time, frequently through the whole period of life; but in Birds and Mammalia they disappear, while they are in the embryo state.