Fig. 362.—Skull and pectoral arch of Conger vulgaris, side view. Ar, Articular; br, branchiostegal rays; ch, ceratohyal; cl, clavicle; cor, coracoid; d, dentary: eot, epiotic; eth, ethmoid; f, frontal; hm, hyomandibular; iop, interoperculum; m, maxilla; n, nasal; op, operculum; p, parietal; pop, praeoperculum; por, praeorbital; ppt, pterygo-palatine; ps, parasphenoid; ptf, post-frontal; ptr, pterygials; q, quadrate; sc, scapula; scl, supra-clavicle; so, supra-occipital; sop, suboperculum; sq, squamosal; uh, urohyal; v, vomer.
Spread over all the seas of the temperate and tropical zones, often descending to the greatest depths, a few entering fresh waters. Many are known to undergo very striking metamorphoses, the pellucid, strongly compressed larvae (Leptocephalus) having long been a puzzle to naturalists.
Nearly 150 recent species are known, of which some 50 are deep-sea forms, occurring down to 2500 fathoms. Scanty fossil remains, referable to recent genera or scarcely different from them, are known from the Eocene of Europe. The Cretaceous genus Urenchelys, from England and the Lebanon, is interesting as representing a more generalised type, the hindmost vertebrae bearing a pair of expanded hypural bones, showing the diphycercal Eels to have been derived from Fishes with a normal caudal fin.
The genera are numerous. The following are the principal:—Anguilla, Simenchelys, Ilyophis, Conger, Coloconger, Congromuraena, Uroconger, Heteroconger, Muraenesox, Nettastoma, Nettophichthys, Saurenchelys, Nettenchelys, Myrus, Myrophis, Derichthys, Chilorhinus, Muraenichthys, Liuranus, Ophichthys, Moringua.
In the first four genera, small, more or less lineal rudimentary scales are embedded in the skin, arranged in small groups, which are placed obliquely at right angles to one another, forming a curious pattern; but these scales are so small that they escape the notice of the superficial observer, hence Eels have been improperly included among the Fishes forbidden as food by the Mosaic prescriptions. In the other genera, including the exclusively marine Conger of our coasts, scales are really absent.
The Common Eel (Anguilla vulgaris) has a very wide distribution, being found over the greater part of Europe, North Africa, Temperate Asia, and perhaps also North America east of the Rocky Mountains, Mexico, and the West Indies. Its record from Australia and New Zealand is probably due to the imperfection of our knowledge of the specific characters. It is not found in the Black Sea nor in the rivers flowing into it, owing, no doubt, to the sulphurous nature of the bottom of the sea, to which, as we now know, these Fish would have to resort for breeding.
The mode of propagation of the Eel long remained a mystery, from the fact that individuals found in fresh water never show ripe genital glands. The idea had been entertained of their being hermaphrodite, and internal parasites had also given rise to the belief in their viviparous nature. The genital glands of the female were first investigated by Rathke in 1838, but it was not until 1874 that those of the male were discovered by Syrski, and shortly after fully described by L. Jacoby, who, in his final contribution to the subject, concluded that Eels need salt water for the development of their organs of generation, and that this development takes place, not near the coast, but further out in deep water.
Fig. 363.—Larva of Common Eel, Leptocephalus brevirostris of Kaup. (After Kaup.)
As a rule it is not until the fifth or sixth year that the Eels go to the sea for the purpose of propagation, which takes place at great depths—at least 200 fathoms. Males have been observed to precede the females. The breeding season over, the Eels do not return to fresh waters, but are believed to die soon after. The eggs were discovered by Raffaele in 1888 in the Gulf of Naples, and shortly after Grassi and Calandruccio finally settled the question of the breeding and development of the Fish from observations made in the Mediterranean. Their conclusions are thus summed up:—"The Common Eel matures in the depths of the sea, where it acquires larger eyes than are ever observed in individuals which have not yet migrated to deep water. The abysses of the sea are its spawning places; its eggs float in the sea water. In developing from the egg, it undergoes a metamorphosis, it passes through a larval form denominated Leptocephalus brevirostris." What length of time the development requires is not yet fully established, since the Leptocephali are rarely found at the surface, most of the specimens studied by Grassi and Calandruccio having been obtained from the stomach of the Sun-Fish (Orthagoriscus mola) in the Straits of Messina; but it is believed that the young Eels or "elvers," which ascend our rivers in such prodigious numbers in spring and summer ("Eel-Fares") are already one year old. Some individuals apparently spend their whole life in fresh waters, but they are barren.[[690]] A specimen was kept in confinement in the family of the French naturalist Desmarest for upwards of 40 years, growing to a length of 4½ feet, being already of large size at the time of its capture. Eels are extremely voracious, and endowed with an extraordinary tenacity of life; they can live for many hours out of the water, and are often met with at night creeping through the grass of meadows from one pond or stream to another.