Swimming is by no means the only form of locomotion in vogue amongst Fishes. A few, like the Angler-Fishes (_Lophius_), habitually use the pectoral fins for crawling about the sea-bottom. The East Indian Goby, _Periophthalmus_, uses its pectoral fins, which are bent at an angle like an elbow-joint, for hopping over sandy flats left bare by the retreating tide. The Flying-Fish (_Exocoetus_), when projected from the water by a stroke of its powerful tail, expands its large pectoral fins, and, using them after the fashion of a parachute, floats through the air for considerable distances before returning to its natural medium. The "Flying Gurnards" (_Dactylopterus_) are also capable of short aerial excursions in a similar fashion. Nor is tree-climbing beyond the province of a Fish, if credit be given to the assertion that the Indian "Climbing-Perch" (_Anabas scandens_) uses its opercular spines for ascending trees. Many freshwater Fishes are known to migrate across land from one pool or river to another, usually during the night. Eels do so by a serpentine or wriggling motion of their long bodies, but in others the pectoral fins seem to be the principal organs used for the purpose, aided, it may be, by a perverted use of the tail. SOUND-PRODUCING ORGANS.—Contrary to popular belief sound-producing or vocal organs are by no means uncommon in Fishes, especially in certain families of Teleosts. It is not always easy, however, to discriminate between involuntary, abnormal, or accidental sounds, and those due to the action of special vocal organs. There are, moreover, some Fishes which observations have shown to utter highly characteristic sounds, although the precise nature of the sound-producing mechanism is at present unknown; while other Fishes appear to possess organs which, on anatomical grounds, are perhaps vocal in function, although nothing is known of the nature of the sounds they emit. Here those organs only will be considered which, either with certainty or with some degree of probability, may be regarded as vocal structures. For most of our knowledge of these interesting structures we are indebted to the researches of Sörensen and Dufossé.[417] {356}[Illustration: FIG. 205.—Stridulating apparatus of _Callomystax gagata_. _is^1_, The first interspinous bone, the lower part of which forms the double file and fits into the interval between the cleft neural spines _ns^4_ and _ns^5_; _is^2_, _is^3_, second and third interspinous bones; _ns^3_, _ns^4_, _ns^5_, neural spines of the third, fourth, and fifth vertebrae; _s^1_, _s^2_, spine-like rays of the dorsal fin; _so_, supra-occipital. (After Haddon.)]