The Labyrinthine Fishes.—An offshoot of the Percomorphi is the group of Labyrinthici, composed of perch-like fishes which have a very peculiar structure to the pharyngeal bones and respiratory apparatus. This feature is thus described by Dr. Gill:
"The upper elements of one of the pairs of gill-bearing arches are peculiarly modified. The elements in question (called branchihyal) of each side, instead of being straight and solid, as in most fishes, are excessively developed and provided with several thin plates or folds, erect from the surface of the bones and the roof of the skull, to which the bones are attached. These plates, by their intersection, form chambers, and are lined with a vascular membrane, which is supplied with large blood-vessels. It was formerly supposed that the chambers referred to had the office of receiving and retaining supplies of water which should trickle down and keep the gills moist; such was supposed to be an adaptation for the sustentation of life out of the water. The experiments of Surgeon Day, however, throw doubt upon this alleged function, and tend to show: (1) that these fishes died when deprived of access to atmospheric air, not from any deleterious properties either in the water or in the apparatus used, but from being unable to subsist on air obtained solely from the water, aerial respiration being indispensable; (2) that they can live in moisture out of the water for lengthened periods, and for a short, but variable period in water only; and (3) that the cavity or receptacle does not contain water, but has a moist secreting surface, in which air is retained for the purpose of respiration. It seems probable that the air, after having been supplied for aerial respiration, is ejected by the mouth, and not swallowed to be discharged per anum. In fine, the two respiratory factors of the branchial apparatus have independent functions: (1) the labyrinthiform, or branchihyal portion, being a special modification for the respiration of atmospheric air, and (2) the gill filaments discharging their normal function. If, however, the fish is kept in water and prevented from coming to the surface to swallow the atmospheric air, the labyrinthiform apparatus becomes filled with water which cannot be discharged, owing to its almost non-contractile powers. There is thus no means of emptying it, and the water probably becomes carbonized and unfit for oxygenizing the blood, so that the whole of the respiration is thus thrown on the branchiæ. This will account for the fact that when the fish is in a state of quiescence, it lives much longer than when excited, whilst the sluggishness sometimes evinced may be due to poisoned or carbonized blood."
Four families of labyrinth-gilled fishes are recognized by Professor Gill; and to these we may append a fifth, which, however, lacks the elaborate structures mentioned above and which shows other evidences of degeneration.
The Climbing-perches: Anabantidæ.—The family of Anabantidæ, according to Gill, "includes those species which have the mouth of moderate size and teeth on the palate (either on the vomer alone, or on both the vomer and palatine bones). To the family belongs the celebrated climbing-fish.
Fig. 303.—The Climbing Perch, Anabas scandens Linnæus. Opercle cut away to show the gill-labyrinth.
"The climbing-fish (Anabas scandens) is especially noteworthy for the movability of the suboperculum. The operculum is serrated. The color is reddish olive, with a blackish spot at the base of the caudal fin; the head, below the level of the eye, grayish, but relieved by an olive band running from the angle of the mouth to the angle of the preoperculum, and with a black spot on the membrane behind the hindermost spines of the operculum.
"The climbing-fish was first made known in a memoir, printed in 1797, by Daldorf, a lieutenant in the service of the Danish East India Company at Tranquebar. Daldorf called it Perca scandens, and affirmed that he himself had taken one of these fishes, clinging by the spine of its operculum in a slit in the bark of a palm (Borassus flabelliformis) which grew near a pond. He also described its mode of progression; and his observations were substantially repeated by the Rev. Mr. John, a missionary resident in the same country. His positive evidence was, however, called into question by those who doubted on account of hypothetical considerations. Even in popular works not generally prone to even a judicious skepticism, the accounts were stigmatized as unworthy of belief. We have, however, in answer to such doubts, too specific information to longer distrust the reliability of the previous reports.
"Mr. Rungasawmy Moodeliar, a native assistant of Capt. Jesse Mitchell of the Madras Government Central Museum, communicated to his superior the statement that 'this fish inhabits tanks or pools of water, and is called Panai feri, i.e., the fish that climbs palmyra-trees. When there are palmyra-trees growing by the side of a tank or pool, when heavy rain falls and the water runs profusely down their trunks, this fish, by means of its opercula, which move unlike those of other fishes, crawls up the tree sideways (i.e., inclining to the sides considerably from the vertical) to a height of from five to seven feet, and then drops down. Should this fish be thrown upon the ground, it runs or proceeds rapidly along in the same manner (sideways) as long as the mucus on it remains.'
"These movements are effected by the opercula, which, it will be remembered, are unusually mobile in this species; they can, according to Captain Mitchell (and I have verified the statement), be raised or turned outwards to nearly a right angle with the body, and when in that position, the suboperculum distends a little, and it appears that it is chiefly by the spines of this latter piece that the fish takes a purchase on the tree or ground. 'I have,' says Captain Mitchell, 'ascertained by experiment that the mere closing of the operculum, when the spines are in contact with any surface, even common glass, pulls an ordinary-sized fish forwards about half an inch,' but it is probable that additional force is supplied by the caudal and anal fins, both of which, it is said, are put in use when climbing or advancing on the ground; the motion, in fact, is described as a wriggling one.