Fig. 284.—Mormyrus oxyrhynchus.
The Mormyri possess a singular organ on each side of the tail, without electric functions, but evidently representing a transitional condition from muscular substance to an electric organ. It is an oblong capsule divided into numerous compartments by vertical transverse septa, and containing a gelatinous substance. The Mormyri differ much with regard to the extent of the dorsal and anal fins, the former sometimes occupying the greater portion of the length of the back, sometimes being much shorter and limited to the tail. In some the snout is short and obtuse, in others long and decurved, with or without appendage.
Of Gymnarchus one species only is known, G. niloticus, which occurs in the Nile and West African rivers, and attains a length of six feet. The form of its body is eel-like, and each jaw is armed with a series of incisor-like teeth. Like Mormyrus, Gymnarchus possesses a pseudo-electric organ, thickest on the tail, tapering in front, and extending nearly to the head. It consists of four membranaceous tubes intimately connected with the surrounding muscles, and containing prismatic bodies arranged in the manner of a paternoster. The air-bladder of Gymnarchus is cellular, very extensible, and communicates with the dorsal side of the œsophagus by a duct possessing a sphincter.
[See Erdl, Münchner Gelehrte Anzeigen, 1846, xxiii., and Hyrtl, Denkschr. Akad. Wiss. Wien. 1856. xii.]
Thirteenth Family—Sternoptychidæ.
Body naked, or with very thin deciduous scales; barbels none. Margin of the upper jaw formed by the maxillary and intermaxillary, both of which are toothed; opercular apparatus not completely developed. Gill-opening very wide; pseudobranchiæ present or absent; air-bladder simple, if present. Adipose fin present, but generally rudimentary. Series of phosphorescent bodies along the lower parts. The eggs are enclosed in the sacs of the ovarium, and excluded by oviducts.
Pelagic and Deep-sea fishes of small size.
Sternoptyx.—Trunk much elevated and compressed, with the trunk of the tail very short. Body covered with a silvery pigment, without regular scales; series of phosphorescent spots run along the lower side of the head, body, and tail. Cleft of the mouth wide, vertical, with the lower jaw prominent. Jaws armed with small teeth. Eyes rather large, and although lateral, directed upwards and placed close together. Ventral fins very small. A series of imbricate scutes runs along the abdomen, forming a kind of serrature. The dorsal fin is short, and occupies about the middle of the length of the fish; it is preceded by the first commencement of the formation of a spinous dorsal, several neural spines being prolonged beyond the dorsal muscle forming a triangular osseous plate. Adipose fin rudimentary; anal short; caudal forked.
These small fishes are now and then picked up in the Mediterranean and Atlantic. According to the dredging-records of the “Challenger,” they and the allied genera Argyropelecus and Polyipnus would descend to depths of respectively 1100 and 2500 fathoms; but the form of their body and their whole organisation render this statement very improbable; they most likely live at a small depth during the daytime, coming to the surface at night, like many Scopelus.