Fig. 100.—Aldrovandia gracilis (Goode & Bean). Guadaloupe Island, West Indies. Family Halosauridæ.
Several fossil Halosauridæ are described from the Cretaceous of Europe and Syria, referred to the genera Echidnocephalus and Enchelurus. Boulenger refers the Lyopomi to the suborder Heteromi.
CHAPTER VII
THE APODES, OR EEL-LIKE FISHES
The Eels.—We may here break the sequence from the Isospondyli to the other soft-rayed fishes, to interpolate a large group of uncertain origin, the series or subclass of eels.
The mass of apodal or eel-like fishes has been usually regarded as constituting a single order, the Apodes (ἄ, without; ποῦς, foot). The group as a whole is characterized by the almost universal separation of the shoulder-girdle from the skull, by the absence of the mesocoracoid arch on the shoulder-girdle, by the presence of more than five pectoral actinosts, as in the Ganoid fishes, by the presence of great numbers of undifferentiated vertebræ, giving the body a snake-like form, by the absence in all living forms of the ventral fins, and, in all living forms, by the absence of a separate caudal fin. These structures indicate a low organization. Some of them are certainly results of degeneration, and others are perhaps indications of primitive simplicity. Within the limits of the group are seen other features of degeneration, notably shown in the progressive loss of the bones of the upper jaw and the membrane-bones of the head and the degradation of the various fins. The symplectic bone is wanting, the notochord is more or less persistent, the vertebral centra always complete constricted cylinders, none coalesced. But, notwithstanding great differences in these regards, the forms have been usually left in a single order, the more degraded forms being regarded as descended from the types which approach nearest to the ordinary fishes. From this view Professor Cope dissents. He recognizes several orders of eels, claiming that we should not unite all these various fishes into a single order on account of the eel-like form. If we do so, we should place in another order those with the fish-like form. It is probable, though not absolutely certain, that the Apodes are related to each other. The loss among them, first, of the connection of the post-temporal with the skull; second, of the separate caudal fin and its hypural support; third, of the distinct maxillary and premaxillary; and fourth, of the pectoral fins, must be regarded as successive phases of a general line of degradation. The large number of actinosts, the persistence of the notochord, the absence of spines, and the large numbers of vertebræ seem to be traits of primitive simplicity. Special lines of degeneration are further shown by deep-sea forms. What the origin of the Apodes may have been is not known with any certainty. They are soft-rayed fishes, with the air-bladder connected by a tube with the œsophagus, and with the anterior vertebræ not modified. In so far they agree with the Isospondyli. In some other respects they resemble the lower Ostariophysi, especially the electric eel and the eel-like catfishes. But these resemblances, mainly superficial, may be wholly deceptive; we have no links which certainly connect the most fish-like Apodes with any of the other orders. Probably Woodward's suggestion that they may form a series parallel with the Isospondyli and independently descended from Tertiary Ganoids deserves serious consideration. Perhaps the most satisfactory arrangement of these fishes will be to regard them as constituting four distinct orders for which we may use the names Symbranchia (including Ichthyocephali and Holostomi), Apodes (including Enchelycephali and Colocephali), Carencheli, and Lyomeri.
Order Symbranchia.—The Symbranchia are distinguished by the development of the ordinary fish mouth, the maxillary and premaxillary being well developed. The gill-openings are very small, and usually confluent below. These fresh-water forms of the tropics, however eel-like in form, may have no real affinity with the true eels. In any event, they should not be placed in the same order with the latter.
The eels of the suborder Ichthyocephali (ιχθύς, fish; κεφαλή, head) have the head distinctly fish-like. The maxillary, premaxillary, and palatines are well developed, and the shoulder-girdle is joined by a post-temporal to the skull. The body is distinctly eel-like, the tail being very short and the fins inconspicuous. The number of vertebræ is unusually large. The order contains the single family Monopteridæ, the rice-field eels, one species, Monopterus albus, being excessively common in pools and ditches from China and southern Japan to India.
The eels of the suborder Holostomi (ὀλός, complete; στόμα, mouth) differ from these mainly in the separation of the shoulder-girdle from the skull, a step in the direction of the true eels. The Symbranchidæ are very close to the Monopteridæ in external appearance, small, dusky, eel-like inhabitants of sluggish ponds and rivers of tropical America and the East Indies. The gill-openings are confluent under the throat. Symbranchus marmoratus ranges northward as far as Vera Cruz, having much the habit of the rice-field eel of Japan and China. The Amphipnoidæ, with peculiar respiratory structures, abound in India. Amphipnous cuchia, according to Günther, has but three gill-arches, with rudimentary lamina and very narrow slits. To supplement this insufficient branchial apparatus, a lung-like sac is developed on each side of the body behind the head, opening between the hyoid and the first branchial arch. The interior of the sac is abundantly provided with blood-vessels, the arterial coming from the branchial arch, whilst those issuing from it unite to form the aorta. Amphipnous has rudimentary scales. The other Holostomi and Ichthyocephali are naked and all lack the pectoral fin.
The Chilobranchidæ are small sea-fishes from Australia, with the tail longer than the rest of the body, instead of much shorter as in the others.