Fig. 168.—Head of young Polypterus. ex.g, External gill of the left side. (From Steindachner.)
Larval Gills.—In early life many Fishes acquire larval gills, either as the result of the precocious growth of the normal gills, or by reason of the development of evanescent structures. In the embryos of Elasmobranchs "external gills," in the form of long filiform processes invested by hypoblast, are developed from the walls of all the branchial clefts, including the spiracles, and protrude outwards for some distance through the external apertures of the clefts (Fig. 167, B). They perhaps facilitate respiration within the egg, as they completely disappear after hatching; but there is also reason for believing that they aid in the absorption of nutriment. Similar gills are present in young Holocephali. In some larval Teleosts, as in certain genera of the Osteoglossidae and Mormyridae (e.g. Heterotis and Gymnarchus)[[306]] these structures are remarkably developed (Fig. 239). The young of the Loach (Misgurnus) and of the Salmon (Salmo) also have the ordinary gill-filaments prolonged externally as filiform structures, which subsequently become reduced to their normal size.[[307]] In its larval state Polypterus[[308]] has a pair of pinnately-fringed ectodermal or cutaneous gills projecting from the lateral surfaces of the head behind and above the external branchial apertures (Figs. 168 and 281). Apparently as an individual peculiarity the right gill has been retained in a specimen of P. congicus so large as 22 cm. in length, although the left one had entirely disappeared.[[309]] Each gill is supplied with blood from the ventral aorta by a vessel which ascends the hyoid arch, and is apparently the representative of the artery supplying the hyoidean hemibranch in Elasmobranchs. The efferent vessel of each gill joins the common trunk formed by the union of the efferent vessels of the normal gills of the same side.
The cutaneous gills of the Dipnoid Protopterus may also be included in the category of larval breathing organs. They consist of three simple unbranched filaments on each side of the head, and, as in Polypterus, are situated at the dorsal extremity of the external gill aperture (Fig. 309). Although usually represented in the relatively young or half grown specimens which, so far, have reached Europe, it is extremely probable that these organs atrophy in older individuals. Similar gills are present in the larval Lepidosiren (Fig. 311), but disappear at a much earlier stage. At no period of its development are larval gills present in Neoceratodus.[[310]]
The Air-Bladder as a respiratory Organ.—In certain Fishes the air-bladder may become subservient to the function of respiration. In Amia and Lepidosteus the internally sacculated and vascular air-bladder is obviously adapted for air-breathing, and there are not wanting observations[[311]] which suggest that the organ is actually used for this purpose after the fashion of a lung. According to Jobert,[[312]] this is also the case with the sacculated air-bladder of certain Brazilian Teleosts, viz. Sudis gigas, Erythrinus taeniatus and E. braziliensis, since these Fishes die of asphyxia when the organ is cut off from communication with the exterior by the ligature of its ductus pneumaticus. It is in the Dipnoi, however, that the air-bladder becomes most completely a true lung. In Neoceratodus[[313]] the lung is probably of the greatest use to the Fish when the rivers are low during the hot season and the water is charged with foul gases from decomposing vegetable matter, and possibly also when the water is filled with sediment in the rainy season. In Protopterus, and more especially in Lepidosiren, the partial atrophy of the gills renders it highly probable that the lungs are the principal breathing organs at all times. Nevertheless, it must be emphasised that in all these Fishes respiration by means of the air-bladder necessarily involves a transit of air to and from that organ through the ductus pneumaticus, and at present nothing is known as to the method by which such inspiratory and expiratory currents can be produced.[[314]]
There is also some experimental evidence for the belief that the air-bladder of some Teleosts may be subsidiary to respiration by acting as a reservoir for the superabundance of oxygen which is taken into the blood through the gills, and subsequently reabsorbed into the blood when the Fish is in water containing relatively little oxygen.[[315]] It is clear, however, that the conditions under which the air-bladder can be used in this way are by no means fully understood, for, under experiment, such Fishes died of asphyxia even though after death the air-bladder still contained upwards of fifty per cent of oxygen.
Accessory Organs of Respiration.—In certain Fishes of peculiar habits, or living under special external conditions, accessory respiratory organs are developed.
Although in this particular instance no special organs are formed, mention may first be made of the singular method of intestinal respiration in vogue in some Teleosts. In one of the Loaches (Misgurnus fossilis),[[316]] air is swallowed and passed along the alimentary canal until it is finally voided at the anus. The mucous membrane of the intestine is extremely vascular, and hence the blood comes into sufficiently intimate relations with the swallowed air to admit of it exchanging carbon dioxide for oxygen. Intestinal respiration also occurs in species of the South American freshwater genera of Siluridae and Loricariidae, Callichthys, Doras, Loricaria, and Plecostomus;[[317]] and in some cases the area of respiratory surface is considerably increased by the development of folds and processes of the intestinal mucous membrane.
In a few tropical Teleosts curious labyrinthiform organs are developed in connexion with certain of the branchial arches, and serve as accessory breathing organs. In the Indian "Climbing Perch" (Anabas scandens),[[318]] of the family Anabantidae, the organ (Fig. 169) consists of three or more concentrically-arranged bony laminae, with wavy, crenulated margins, attached by a common bony base to the upper extremity of the fourth branchial arch, and enclosed in a special dorsal enlargement of the branchial cavity. The vascular membrane which invests the laminae is abundantly supplied with venous blood by a branch of the fourth afferent branchial artery, the equivalent efferent vessel joining the dorsal aorta. Essentially similar organs are found in several genera of Osphromenidae (e.g. Polyacanthus, Osphromenus, and Trichogaster). A simpler form of respiratory organ of somewhat the same type occurs in the Indian family Ophiocephalidae.[[319]] In these Fishes there is, on each side, an accessory branchial cavity, situated above that which contains the gills, but freely communicating with it (Fig. 170). The cavity is lined by a thickened and puckered vascular membrane, but otherwise contains no special respiratory structures.
