Fam. 1. Symbranchidae.—Post-temporal well developed, forked, attached to the skull. Inhabitants of the fresh or brackish waters of South-Eastern Asia, Tropical America, Australia, and Tasmania. Three genera are known: Symbranchus, with two species from India and the Malay region, and one from Central and South America; Monopterus, with a single species from China, Japan, and the Malay region; and Chilobranchus, with two species from Australia and Tasmania. Although the South American Symbranchus has been observed to live in marshes which periodically dry up, the Fish burying itself in the mud like a Lepidosiren, the branchiae are fully developed on the four branchial arches. In Monopterus, of similar habits, the branchial laminae are rudimentary, and on three arches only. No accessory breathing organ is known to exist.
Fig. 361.—Monopterus javanensis. Lower view of head, showing gill-opening (go): lower view of middle praecaudal vertebrae: and side view of skull and pectoral arch. ar, Articular; br, branchiostegal rays; bra, branchial arches; cl, clavicle; d, dentary; eot, epiotic; eth, ethmoid; f, frontal; hm, hyomandibular; iop, interoperculum; m, maxilla; mpt, metapterygoid; n, nasal; op, operculum; p, parietal; pm, praemaxilla; pop, praeoperculum; ppt, pterygopalatine; ptte, post-temporal; q, quadrate; scl, supra-clavicle; so, supra-occipital; sop, suboperculum; sq, squamosal; sy, symplectic.
Fam. 2. Amphipnoidae.—Post-temporal absent, the shoulder-girdle free from the skull. The Cuchia, Amphipnous cuchia, the sole representative of this family, an inhabitant of the fresh and brackish waters of India and Burma, growing to two feet in length, is remarkable for the presence of a respiratory air-sac on each side of the neck behind the head, communicating with the gill-cavity. Of the three branchial arches the second alone possesses gill-filaments; the third supports, in their place, a thick and semi-transparent tissue; the principal organs of respiration are two small bladders, resembling the posterior portions of the lungs of snakes, which the animal has the power of filling with air immediately derived from the atmosphere. Although covered over by the common integuments, these bladders present externally, when inflated, two protuberances of a round shape. Of the whole volume of blood contained in the branchial artery, one-third passes through the gills and respiratory bladders, whilst the other two-thirds are conveyed directly from the heart to the aorta without being exposed to the action of the air.[[688]] This amphibious Fish, when in the water, constantly rises to the surface for the purpose of respiration, and it is often found lying in the grassy sides of ponds after the manner of Snakes.
Sub-Order 4. Apodes.
Air-bladder, if present, communicating with the digestive tract by a duct. Praemaxillaries absent; the maxillaries, if present, separated on the median line by the coalesced ethmoid and vomer. Pectoral arch, if present, not connected with and remote from the skull; mesocoracoid arch absent. Fins without spines, the ventrals absent. Anterior vertebrae distinct, without Weberian ossicles.
The Apodes or Eels are elongate, serpentiform Fishes with naked skin, or with minute scales imbedded in the skin, the opercular bones small and completely hidden under the integument; narrow or minute gill-openings; the vertical fins, if present, confluent behind or separated by the projecting tip of the tail. The pterygo-palatine arch is often reduced or absent, and there is no distinct symplectic; the supraoccipital bone is small, separated from the frontals by the parietals, which meet on the middle line. The vertebrae are very numerous (up to 225), and the praecaudals bear strong parapophyses, to which short, slender ribs may be attached; epineurals are sometimes present. The five families into which this sub-order is divided show remarkable degrees of simplification of the skull, through reduction or loss of either the maxillary or the pterygo-palatine arches.
There has been much difference of opinion in the determination of the bones of the upper jaw in these Fishes. Cuvier regarded the lateral bones of the upper jaw as praemaxillaries, Owen and Richardson as palatines (at least in Muraenas), whilst Peters and most recent authors have identified them throughout the order as maxillaries.[[689]] The conclusion I have come to from the examination of numerous skulls belonging to various genera, is that the praemaxillaries have disappeared in all, whilst the maxillaries have persisted in the true Eels (Anguillidae) and disappeared in the Muraenidae, their place being taken by the fused palato-ectopterygoids, which may even join the mandibular suspensorium. The vestigial bone, regarded by Jacoby as the pterygoid in Muraena helena, may be identified as the meta-pterygoid, and therefore does not disprove the homology, here suggested, of the other elements of the palate.
Fam. 1. Anguillidae.—Maxillaries present, separated on the median line by the ethmo-vomer; palato-pterygoid bone present, connected with the hyomandibular and quadrate; gill-clefts separate, opening into the pharynx by wide slits; tongue present; vent far removed from the head.
