Fig. 117.—Gulper-eel, Gastrostomus bairdi Gill & Ryder. Gulf Stream.
Order Lyomeri, or Gulpers.—Still more aberrent and in many respects extraordinary are the eels of the order or suborder Lyomeri (λυός, loose; μέρος, part), known as "Gulpers." These are degenerate forms, possibly degraded from some conger-like type, but characterized by an extreme looseness of structure unique among fishes. The gill-arches are reduced to five small bars of bone, not attached to the skull, the palatopterygoid arch is wholly wanting, the premaxillaries are wanting, as in all true eels, and the maxillaries loosely joined to the skull. The symplectic bone is wanting, and the lower jaw is so hinged to the skull that it swings freely in various directions. In place of the lateral line are singular appendages. Dr. Gill says of these fishes: "The entire organization is peculiar to the extent of anomaly, and our old conceptions of the characteristics of a fish require to be modified in the light of our knowledge of such strange beings." Special features are the extraordinary size of the mouth, which has a cavity larger than that of the rest of the body, the insertion of the very small eye at the tip of the snout, and the relative length of the tail. The whole substance is excessively fragile as usual with animals living in great depths and the color is jet black. Three species have been described, and these have been placed in two families, Saccopharyngidæ, with the trunk (gill-opening to the vent) much longer than the head, and Eurypharyngidæ, with the trunk very short, much shorter than the head. The best-known species is the pelican eel (Eurypharynx pelacanoides), of the coast of Morocco, described by Vaillant in 1882. Gastrostomus bairdi, very much like it, occurs in the great depths under the Gulf Stream. So fragile and so easily distorted are these fishes that it is possible that all three are really the same species, for which the oldest name would be Saccopharynx ampullaceus. Of this form four specimens have been taken in the Atlantic, one of them six feet long, carried to the surface through having swallowed fishes too large to be controlled. To be carried above its depth in a struggle with its prey is one of the greatest dangers to which the abysmal fishes are subject.
Order Heteromi.—The order of Heteromi (ἑτερός, different; ὤμος, shoulder), or spiny eels, may be here noticed for want of a better place, as its affinities are very uncertain. Some writers have regarded it as allied to the eels; some have placed it among the Ganoids. Others have found affinities with the sticklebacks, and still others with the singular fresh-water fishes called Mastacembelus. The Heteromi agree with the eels, as well as with Mastacembelus, in having the scapular arch separate from the cranium. Unlike all the true eels, most of the species have true dorsal and anal spines, as in the Percesoces and Hemibranchii. The ventral fins, when present, are abdominal and each with several spines in front, a character not found among the Acanthopteri. There is no mesocoracoid.
The air-bladder has a duct, and the coracoids, much as in the Xenomi, are reduced to a single lamellar imperforate plate. The two groups have little else in common, however, and this trait is possibly primitive in both cases, more likely to have arisen through independent degeneration. The separation of the shoulder-girdle doubtless indicates no affinity with the eels, as the bones of the jaws are quite normal. Two families are known, both from the deep sea, besides an extinct family in which spines are not developed.
The Notacanthidæ are elongate, compressed, ending in a band-shaped, tapering tail; the back has numerous free spines and few or no soft rays, and the mouth is normal, provided with teeth. The species of Notacanthus are few and scantily preserved. Those of Macdonaldia are more abundant. Macdonaldia challengeri is from the North Pacific, being once taken off Tokio. The extinct family of Protonotacanthidæ differs in the total absence of dorsal spines and fin-rays; the single species, Pronotocanthus sahel-almæ, originally described as a primitive eel, occurs in the Cretaceous of Mount Lebanon.
The Lipogenyidæ have a round, sucker-like mouth, with imperfect lower jaw, but are otherwise similar. Lipogenys gilli was dredged in the Gulf Stream.
Fig. 118.—Notacanthus phasganorus Goode & Bean. Grand Banks.
Dr. Boulenger has recently extended the group of Heteromi by the addition of the Dercetidæ, Halosauridæ (Lyopomi), and the Fierasferidæ. We can hardly suppose that all these forms are really allied to Notacanthus.