The Plectognaths are mostly denizens of the warmer seas, but the pig-faced trigger-fish of the Mediterranean (Balistes capriscus) has been caught three times in the British waters since 1827, and the short sun-fish or molebut, though occurring but occasionally, may be said to have been taken from John o' Groat's to the Land's End. It grows to an immense size, often attaining the diameter of four feet, sometimes even double that size, and occasionally weighing from 300 to 500 pounds. When observed in our seas, the sun-fishes have generally appeared as though they were dead or dying, floating lazily along on one side and making little or no attempt to escape. It is to be presumed that in more congenial waters they evince a greater degree of liveliness.
The order of the Lophobranchii is in many respects too curious and interesting to be passed over in silence. Here the gills, instead of being as usual ranged like the teeth of a comb, are clustered into small filamentous tufts placed by pairs along the branchial arches; the face projects into a long tubular snout, having the mouth either at its extremity, as in the Hippocampus and in the Pipe-fishes, or at its base, as in the Pegasus of the Indian seas; and the body is covered with shields or small plates, which often give it an angular form, and encase it as it were in jointed armour. But the most interesting feature of their economy is the pouches in which the males of the most characteristic genera carry the eggs until they are hatched. In the hippocampi this provision for the safety of the future generation, which strongly reminds one of the kangaroo or the opossum, forms a perfect sack, opening at its commencement only; in the pipe-fishes it is closed along its whole length by two soft flaps folding over each other. Another peculiarity of these interesting little fishes is the independent motion of their eyes, the one glancing hither and thither while its fellow remains motionless, or looks in different directions. This phenomenon of double vision, which was long supposed to be peculiar to the chameleon, is, however, not confined to this singular reptile or to the hippocampi and pipe-fishes, but has been found by Mr. Gosse to exist likewise in the Little Weever (Trachinus vipera), in the Suckers (Lepidogastri), a small family remarkable for the power they possess of attaching themselves to stones or rocks by means of an adhesive disk on the under surface of their bodies, and in several other fishes.
When imprisoned in an aquarium, few subjects of the deep display more intelligence or afford more entertainment than the little Hippocampus brevirostris, or Sea-Horse.
Sea-Horse.
"While swimming about," says Mr. Lukis,[P] "it maintains a vertical position, but the tail, ready to grasp whatever meets it in the water, quickly entwines itself in any direction round the weeds, and, when fixed, the animal intently watches the surrounding objects, and darts at its prey with great dexterity. When two of them approach each other, they often twist their tails together, and struggle to separate or attach themselves to the weeds; this is done by the under part of their cheeks or chin, which is also used for raising the body when a new spot is wanted for the tail to fasten upon afresh."
[P] Yarrell, "British Fishes," 3rd edition, vol. ii. p. 396.
"In captivity," says Mr. Gosse, "the manners of the Worm Pipe-Fish (Syngnathus lambriciformis), the smallest of our native species, are amusing and engaging. Its beautiful eyes move independently of each other, like those of the chameleon, and another point of resemblance to that animal our little pipe-fish presents in the prehensile character of its tail. It curves just the tip of this organ laterally round the stem or frond of some sea-weed and holds on by this half-inch or so, while the rest of its body roves to and fro, elevating and depressing the head and fore parts, and throwing the body into the most graceful curves. All the motions of the Pipe-fish manifest much intelligence. It is a timid little thing, retiring from the side of the glass at which it had been lying when one approaches, and hiding under the shadow of the sea-weeds, which I have put in, both to afford it shelter, and also to supply food in the numerous animalcules that inhabit these marine plants. Then it cautiously glides among their bushy fronds, and from under their shelter peeps with its brilliant eyes at the intruder as if wondering what he can be, drawing back gently at any alarming motion. In swimming, it is constantly throwing its body into elegant contortions and undulations; often it hangs nearly perpendicular with the tail near the surface; now and then it butts against the side of the vessel with reiterated blows of its nose, as if it could not make out why it should not go forward where it can see no impediment. Now it twists about as if it would tie its body into a love knot, then hangs motionless in some one of the 'lines of beauty' in which it has accidentally paused."
The family of the Pleuronectidæ or Flat-fishes recommends itself to our notice as much by the singularity of its form as by its usefulness to man. "The want of symmetry," says Yarrell, "so unusual in vertebrated animals, is the most striking and distinctive character of these fishes: the twisted head with both eyes on the same side, one higher than the other, not in the same vertical line, and often unequal in size; the mouth cleft awry, and the frequent want of uniformity in those fins that are in pairs, the pectoral and ventral fins of the under side being generally smaller; and the whole of the colour of the fish confined to one side, while the other side remains white, produce a grotesque appearance: yet a little consideration will prove that these various and seemingly obvious anomalies are perfectly in harmony with that station in nature which an animal possessing such conformation is appointed to fill.
"As birds are seen to occupy very different situations, some obtaining their food on the ground, others on trees, and not a few at various degrees of elevation in the air, so are fishes destined to reside in different depths of water. The flat-fishes and the various species of skate are, by their depressed form of body, admirably adapted to inhabit the lowest position, where they occupy the least space among their kindred fishes."