The fishes belonging to the single family of this division, Gobiesocidæ, are strictly marine but littoral fishes. They are scattered over the temperate zones of both hemispheres, and more numerous than between the Tropics. All are of small or very small size.
Fig. 232.—Gobiesox cephalus.
The adhesive disk consists of an anterior and posterior division. In some of the genera the posterior division has no free anterior margin, the teeth being either all conical, as in Chorisochismus (Cape of Good Hope) and Cotylis (Red Sea and Indian Ocean); or incisor-like in both jaws, as in Sicyases (coast of Chili and West Indies); or incisor-like at least in the lower jaw, as in Gobiesox (West Indies and Pacific coasts of South America). In other genera the posterior portion of the adhesive disk has a free anterior margin. Only one of these genera has incisor-like teeth, viz. Diplocrepis from New Zealand. In the remaining genera, Crepidogaster (from Tasmania and South Australia), Trachelochismus (from New Zealand and the Fiji Islands), Lepadogaster, and Leptopterygius, the teeth are very small and fine. The two last genera are European, and Lepadogaster at least is common on the Southern British coasts. The three species known as British—L. gouanii, L. candollii, and L. bimaculatus—are prettily coloured, but subject to great variation.
Fig. 233.—Diplocrepis puniceus.
Fifteenth Division—Acanthopterygii Channiformes.
Body elongate, covered with scales of moderate size; no spine in any of the fins; dorsal and anal long. No superbranchial organ, only a bony prominence on the anterior surface of the hyomandibular.
These fishes belong to the single family Ophiocephalidæ, Freshwater-fishes characteristic of the Indian region, which, however, have found their way into Africa, where they are represented by one or two species. Thirty-one species are known altogether, most of which are extremely abundant; some attain to a length of more than two feet. Like other tropical freshwater fishes, they are able to survive droughts, living in semi-fluid mud, or lying in a torpid state below the hard-baked crusts of the bottom of a tank from which every drop of water has disappeared. Respiration is probably entirely suspended during the state of torpidity, but whilst the mud is still soft enough to allow them to come to the surface, they rise at intervals to take in a quantity of air, by means of which their blood is oxygenised. This habit has been observed in some species to continue also to the period of the year in which the fish lives in normal water, and individuals which are kept in a basin and prevented from coming to the surface and renewing the air for respiratory purposes, are suffocated. The particular manner in which the accessory branchial cavity participates in respiratory functions is not known. It is a simple cavity, without an accessory branchial organ, the opening of which is partly closed by a fold of the mucous membrane.
