In the sub-kingdom mollusca, we find several modifications of gills. In the lanzelli branchiata, or common bivalves, there are, as a general rule, two gills on each side. Here the gills are internal highly vascular folds of the mantle lining the valves, and are strengthened by delicate jointed filaments, which support several rows of vibratile cilia, whose constant motion gives rise to regular respiratory currents. This form of gill may be readily examined in the oyster or common muscle. In the branchiferous gas teropoda, the form and position of the gills are very variable. In the nudibrandliata (see Alder and Hancock's splendid monograph on this order as occurring in the British. seas), they are disposed, as their name implies, without any protection, over various parts of the body, where they often form beautiful tufts of delicate leaf-like or arbores cent appendages, as may be seen in doris (q.v.). The highest and most numerous sub division of the branchiferous gasteropoda—the order pectinibranddata—derives its name from the peculiar comb-like arrangement of its gills, which have a special cavity at the fore part of the back, caused by an arching of the mantle. Finally, in the highest class of mollusks—the eephalopoda—the gills are the organs used for classification; there being two orders—viz., the tetrabranchiata, with four gills, and the dibranchiata, with two gills.
In the article FISIIES, the gills are of necessity briefly noticed, but all details regard ing them have been postponed to this article. The remarks on the gills of fishes are condensed from prof. Owen's Anatomy of the Vertebrates, vol. i. pp. In the Cyelostomi, which, if we except the lancelet, constitute the lowest order of fishes, and include the genera myrine and petromyzon, of which the hag and lamprey are exam ples, the branchim or gills ilk ,saecifofni, with external spiracles, and six or seven in number, on each side. Each gill-sac receives its proper artery either from the branchial artery or one of its branches. "The leading condition of the gills in other fishes may be understood," says prof. Owen, " by supposing each compressed sac of a myxiue to be split through its plane, and each half to be glued by its outer smooth side to an inter mediate septum, which would then support the opposite halves of two distinct sacs, and expose their vascular mucous membrane to view. If the septum be attached by its entire margin, the condition of the gill in the plagiostomi (sharks, dog-fish, rays, skates, etc.) is effected. If the septum be liberated at the outer part of its circumference, and the vascular surfaces are produced into pectinated lamelligerous processes, tufts or filaments proceeding from the free arch, the gill of an ordinary osseous fish is formed. Such a gill is the homologue, not of a single gill-sac, but of the contiguous halves of two distinct gill-sacs, in the myxines. Already, in the lampreys, the first stage of this bipartition may be seen, and the next stage in the sharks and rays; consequently, in these fishes a different artery goes to the anterior branchial surface of each sac or fissure from that which supplies the posterior bronchial surface of the same fissure; whilst one bronchial artery is appropriated to each supporting septum or arch between the fissures, as it is to the liberated septum or branchial arch in the ordinary osseous fishes."— Anatomy of Vertebrates, vol. 1. p. 476.
The lampreys, myxinoids, and plagiostomes (sharks and rays) are termed fishes with "fixed gills," because in them each supporting septum of the anterior and posterior branchial mucous surfaces is attached to the pharyngeal and dermal integument by its entire outer margin, and the streams of water flow out by the same number of fissures in the skin as those by which they enter from the pharynx. In the osseous and in the ganoid fishes there arc "free gills," the outer border of the supporting brauchial arch being unattached to the skin, and playing freely backwards and forwards, with its surfaces, in a common gill-cavity, which has a single outlet, usually in the form of a. vertical fissure.
In the myxinoids (as the hag) there are six or seven bronchial sacs on either side, and their outlets are produced into short tubes, which open into a longitudinal canal, directed backwards, and discharging its contents by an orifice near the middle line of the ventral surface; between the two outlets is a third larger one, which communicates, by a short duct, with end of the oesophagus, and admits the water, which passes from that tube by the lateral orifices leading into the branchial sacs. These sacs, which
are developed from the oesophagus, and which may be regarded as the simplest form of piscine gill, have a' highly vascular, but not a ciliated, mucous membrane, which is arranged in radiating primary and secondary folds, so as to increase the surface. In the lampreys, there is a further separation of the respiratory from the digestive tract, for each internal blind duct communicates with a median canal, beneath and distinct from the oesophagus.
In all the higher fishes, the inlets to the branchial interspaces lie on each side of the fauces, and are equal in number with the interspaces; while, except in the plagiostomes, there is only one outlet on each side. These outlets vary extremely in size, being relatively largest in the herring and mackerel families, and smallest in the eels and lophioid fishes (as the Angler, q.v.). The length of time that different fishes can exist out of water depends on the modifications for retaining water in the bronchial chain bers. As a general rule. the chamber is largest when the outlet is smallest, as in the eels, blennies, and lophioids, and these are the fishes that survive the longest out of water, except in such cases as the climbing perch (q.v.) or anabas, in which the branchial apparatus possesses complex labyrinthic appendages. The main object of the. gills of fishes being to expose the venous blood, in very thin-walled vessels, to streams. of water, the bronchial arteries rapidly sub-divide into capillaries, which coustitute a net work in one layer, supported by an elastic plate, and covered by a tesselated but non: ciliated epithelium. This covering and the capillary wall are so thin as to admit free interchange to take place between the blood, loaded with carbonic acid, on the one hand, and the aerated water on the other. The extent of respiratory surface is increased in various ways, of which, by far the most common is, " by the production of the capillary supporting plates from each side of long, compressed, slender, pointed processes, extending, like the teeth of a comb, but iu a double row, from the convex side of each branchial arch." The number of vascular plates or lamellm attached to each branchial process has been estimated at 135 in the carp, 700 in the eel, 1000 in the cod, 1400 in the salmon, and 1600 in the sturgeon.
We now pass on to the consideration of these organs in amphibia or batrachia. In the lower or perennibranchiate members of this order, the gills exist permanently, but in the great majority they are mere temporary organs. The subject is briefly noticed in. the article BATRACIIIA ; but one or two additional observations may be made. In the newt (triton), a little animal common in most parts of England, and readily kept in a. vessel of fresh water, three pairs of external gills are developed, at first as simple filaments, each with a capillary loop, but speedily expanding and giving off looplets. The gill is covered with ciliated epithelium. which loses the cilia before the absorption. of the organ, and this takes place after a few days of larval existence. In the larval frog, the gills, which are on a simpler plan, diminish about the 4t1i, and disappear on day. The parts of the branchial, frainewbrk Which supports Ow deciduous gills never get beyond the cartilaginous stage. They thus readily shrink, and become more internal as the head increases in size. As the gills of the perennibranchiate amphibians, in all essential points, resemble those already described, it is unnecessary to notice them. The present article must be regarded as supplementary to RESPIRATION, ORGANS AND PROCESS OF, in which the comparative anatomy of the subject was altogether omitted.