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 {293}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. [Illustration: FIG. 169.—Labyrinthiform organ of _Anabas scandens_, exposed by the removal of the greater part of the operculum. _b.a′_, First branchial arch; _l.o_, labyrinthiform organ; _op_, operculum; _sb.c_, supra-branchial cavity.] [Illustration: FIG. 170.—Supra-branchial cavities of _Ophiocephalus_. Ventral view, as seen after the removal of the ventral halves of the branchial arches, _b.a^{1-4}_, The first four branchial arches; _o.c_, roof of oral cavity; _oes_, oesophagus; _p.t_, pharyngeal teeth; _sb.c_; left supra-branchial cavity; _v.f_, folds of the lining membrane of the cavity.]