b. Plotosina.
Plotosus.—A short dorsal fin in front, with a pungent spine; a second long dorsal coalesces with the caudal and anal. Vomerine teeth molar-like. Barbels eight or ten; one immediately before the posterior nostril, which is remote from the anterior, the latter being quite in front of the snout. Cleft of the mouth transverse. Eyes small. The gill-membranes are not confluent with the skin of the isthmus. Ventral fins many-rayed. Head depressed; body elongate.
Fig. 258.—Mouth of Cnidoglanis megastoma, Australia.
Three species are known from brackish waters of the Indian Ocean freely entering the sea. Plotosus anguillaris is distinguished by two white longitudinal bands, and is one of the most generally distributed and common Indian fishes.—Copidoglanis and Cnidoglanis are two very closely allied forms, chiefly from rivers and brackish waters of Australia. None of these Siluroids attain to a considerable size. Chaca, from the East Indies, belongs likewise to this sub-family.
Fig. 259.—Cnidoglanis microcephalus.
II. Siluridæ Heteropteræ.—The rayed dorsal fin is very little developed, and, if it is present, it belongs to the abdominal portion of the vertebral column; the adipose fin is exceedingly small or absent. The extent of the anal is not much inferior to that of the caudal vertebral column. The gill-membranes overlap the isthmus, remaining more or less separate: Silurina.
Saccobranchus.—Adipose fin none; dorsal very short, without pungent spine, placed above the ventrals. Cleft of the mouth transverse, anterior, of moderate width; barbels eight. Eyes rather small. The upper and lateral parts of the head osseous or covered with a very thin skin. Gill-cavity with an accessory posterior sac, extending backwards between the muscles along each side of the abdominal and caudal portions of the vertebral column. Ventrals six-rayed.
Small fishes from East Indian rivers; four species are known. The lung-like extension of the branchial cavity receives water, and is surrounded by contractile transverse muscular fibres by which the water is expelled at intervals. The vessels of the sac take their origin in the last branchial artery, and pass into the aorta.