From a physiological point of view, this Family may be divided into carnivorous forms, with short digestive tract, and phytophagous or limnophagous ones, in which the intestine forms numerous coils. To the first division belong the living genera Cyprinodon, Characodon, Tellia, Haplochilus, Fundulus, Rivulus, Cynolebias, Orestias, Empetrichthys, Jenynsia, Pseudoxiphophorus, Belonesox, Gambusia, Anableps, among existing forms, and the fossil genera Prolebias (Oligocene and Miocene) and Pachylebias (Miocene); to the second, the living genera Poecilia, Mollienesia, Platypoecilus, and Girardinus. Procatopus, a near ally of Haplochilus, recently discovered in South Cameroon, is remarkable for having the ventral fins inserted far forward, below the pectoral fins.
These are small or very small Fishes,[[694]] only a few reaching a length of about a foot, confined to fresh or brackish waters, recognisable externally by the flat head with protractile mouth, the usually large scales, and the absence of a well-developed lateral line. The teeth vary much in shape: cardiform, villiform, or compressed, and bi- or tri-cuspid; the palate is either toothless, or teeth are present on the vomer. About 200 species are known, mostly from the American continent, only about 30 being known from other parts of the world, viz. Southern Europe, Southern Asia and Japan, and Africa. In many species the sexes are dissimilar, the female being larger and less brilliantly coloured, with smaller fins; the anal fin of the male may be modified into an intromittent organ by means of which internal fertilisation takes place, the ova developing in a sort of uterus, which the young leave in a more or less advanced stage of growth. The most curious of the Cyprinodontids is the genus Anableps, of Central and South America, surface-swimming Fishes, the strongly projecting eyes of which are divided by a horizontal band of the conjunctiva into an upper part adapted for vision in the air, and a lower for vision in the water, and the pupil is divided into two parts by a constriction; the larger species grows to the length of a foot.
Fig. 374.—Anableps tetrophthalmus, male, ½ nat. size.
Fig. 375.—Distribution of the Cyprinodontidae.
Fam. 12. Amblyopsidae.—Mouth scarcely protractile, the maxillaries excluded from the oral border; teeth small, in jaws and palate, and on the pharyngeal bones. Praecaudal vertebrae with very strong parapophyses, bearing the ribs on their upper surface; epipleurals inserted on the ribs. Ventral fins rudimentary or absent. Vent jugular, close to the gill-clefts. Air-bladder present.
Fig. 376.—A, Chologaster cornutus, and B, Amblyopsis spelaea, nat. size. (After Jordan.)
Small ovoviviparous Fishes, closely related to, and evidently derived from, the Cyprinodontids, measuring from 1 to 5 inches, inhabiting ditches and small streams, or confined to subterranean waters of limestone caves, in the United States east of the Rocky Mountains. Six species, referable to three genera, are known. In Chologaster, the eyes are well developed and the body is coloured. C. cornutus inhabits the lowland streams and swamps of the South Atlantic States, from Virginia to Florida; C. agassizii is found in the underground streams of Kentucky and Tennessee; and C. papilliferus occurs under stones in the springs of south-western Illinois. Amblyopsis and Typhlichthys, which are evidently derived from the former, or from forms closely related to it, have the eyes rudimentary and more or less concealed under the skin, and the body is colourless. Amblyopsis spelaea is widely distributed in the caves east of the Mississippi, both north and south of the Ohio River; it is common in the River Styx of the Mammoth Cave. Typhlichthys subterraneus is found with the latter species in the caves east of the Mississippi, but is confined to the south side of the Ohio River, whilst T. (Troglichthys) rosae is found in the caves west of the Mississippi River. Of Amblyopsis spelaea, the late Professor Cope has observed: "If these Amblyopses be not alarmed, they come to the surface to feed, and swim in full sight, like white aquatic ghosts. They are then easily taken by the hand or net, if perfect silence is preserved, for they are unconscious of the presence of an enemy except through the medium of hearing; this sense, however, is evidently very acute, for at any noise they turn suddenly downwards, and hide beneath stones, etc., on the bottom." Dr. Garman thinks, on the contrary, that such a sense can hardly be developed in recesses where we are accustomed to think any sounds other than those made by the rippling or dripping water are almost unknown, and that it is through the sense of touch, and not through hearing, that the Fish is disturbed. In fact, the head is provided with a great number of tactile papillae, arranged in transverse ridges, provided with nervous filaments, which evidently compensate the loss of the visual organ.[[695]]