This sub-order is distinguished by the fact that the fins are entirely supported by "soft" rays, and never possess "spiny" rays; whilst the ventral fins are either wanting, or, if present, are placed under the throat, beneath or in advance of the pectorals, and supported by the pectoral arch. The swim-bladder may be wanting, but when present it does not communicate with the oesophagus by a duct.
As in the preceding order, the Anacanthini are divided into two groups, distinguished by the presence or absence of the ventral fins. In the first of these groups (Apoda) are only a few fishes, of which one of the most familiar examples is the little Sand-eel (Ammodytes lancea), which occurs on all our coasts. In the second group (Sub-brachiata), in which ventral fins exist, are the two important families of the Gadidae and Pleuronectidae. The Gadidae or Cod family, comprising the Haddock, Whiting, Ling, and Cod itself, is of great value to man, most of its members being largely consumed as food. In the Pleuronectidae or Flat-fishes are comprised the Sole, Plaice, Turbot, Halibut, Brill, and others, in all of which there is a very curious modification in the form of the body. The body, namely, in all the Flat-fishes (fig. 263) is very much compressed from side to side, and is bordered by long dorsal and anal fins. When young, the body is symmetrical, the eyes are bilaterally situated, and the animal swims in a vertical position. Soon, the habit of lying on one side (sometimes the right, but more commonly the left, side) is commenced, and then the eye upon the lower side is gradually translated to the upper side of the head; this translation being effected by an actual movement of the lower eye, or by its passing through the at that time soft tissues of the head, a partial twisting of the cranial bones assisting to bring about the final result. When adult, both eyes are situated upon one side of the head (fig. 263), and the fish now keeps this side uppermost, and is dark-coloured on this aspect; whilst the opposite side, on which it rests, is white. From this habit of the Flat-fishes of resting upon one flat surface, the sides are often looked upon as the dorsal and ventral surfaces of the body. This, however, is erroneous, as they are shown by the position of the paired fins to be truly the lateral surfaces of the body. The mouth has its two sides unequal, the pectorals are rarely of the same size, the ventrals look like a continuation of the anal fin, and the branchiostegal rays are six in number.
Fig. 263. - Pleuronectidae. Rhombus punctatus. Natural size (after Gosse).
This sub-order is characterised by the fact that one or more of the first rays in the fins are in the form of true, unjointed, inflexible, "spiny" rays. The exoskeleton consists, as a rule, of ctenoid scales. The ventral fins are generally beneath or in advance of the pectorals, and the duct of the swim-bladder is invariably obliterated.
This sub-order comprises two families:
a. The Pharyngognathi, in which the inferior pharyngeal bones are an-chylosed so as to form a single bone, which is usually armed with teeth. The family is not of much importance, the only familiar fishes belonging to it being the "Wrasses" (Cyclolabridae).
b. The Acanthopteri veri, characterised by having always spiny rays in the first dorsal fin, and usually in the first rays of the other fins, whilst the inferior pharyngeal bones are never anchylosed into a single mass. This family includes many subordinate groups, and may be regarded as, on the whole, the most typical division of the Teleostean fishes. It will not be necessary, however, to do more than mention as amongst the more important fishes contained in it, the Perch family (Percidae), the Mullets (Mugi-lidae), the Mackerel family (Scomberidae), the Gurnards (Sclerogenzdae), the Gobies (Gobiidae), the Blennies (Blenniidae), and the Anglers (Lophiidae). The Percidae form by far the most important member of this group, and are distinguished by having ctenoid scales, the operculum and prae-operculum variously armed with spines, teeth on the vomer and palate as well as on the jaws, and the branchiostegal rays from five to seven in number.