The Flatfishes.—Perhaps the most remarkable offshoot from the order of spiny-rayed fishes is the great group of flounders and soles, called by Bonaparte Heterosomata (ἔτερός, differing; σῶμα, body). The essential character of this group is found in the twisting of the anterior part of the cranium, an arrangement which brings both eyes on the same side of the head. This is accompanied by a great compression of the body, as a result of which the flounders swim horizontally or lie flat on the sand. On the side which is uppermost both eyes are placed, this side being colored, brown or gray or mottled. The lower side is usually plain white. In certain genera the right side is uppermost, in others the left. In a very few, confined to the coast of California, the eyes are on the right or left side indifferently.

The process of the twisting of the head has been already described (see p. 174, Vol. I). The very young have the body translucent and symmetrical, standing upright in the water. Soon the tendency to rest on the bottom sets in, the body leans to left or right, and the lower eye gradually traverses the front of the head to the other side. This movement is best seen in the species of Platophrys, in which the final arrangement of the eyes is a highly specialized one.

In some or all of the soles it is perhaps true that the eye turns over and pierces the cranium instead of passing across it. This opinion needs verification, and the process should be studied in detail in as many species as possible. The present writer has seen it in species of Platophrys only, the same genus in which it was carefully studied by Dr. Carlo F. Emery of Bologna. In the halibut, and in the more primitive flounders generally, the process takes place at an earlier stage than in Platophrys.

Optic Nerves of Flounders.—In the Bulletin of the Museum of Comparative Zoology (Vol. XL, No. 5) Professor George H. Parker discusses the relations of the optic nerves in the group of flounders or flatfishes.

In the bony fishes the optic nerves pass to the optic lobes of the brain, the one passing to the lobes of the opposite side simply lying over the other, without intermingling of fibers, such as takes place in the higher vertebrates and in the more primitive fishes.

According to Parker's observations, in ordinary bony fishes the right nerve may be indifferently above or below the other. In 1000 specimens of ten common species, 486 have the left nerve uppermost and 514 the right nerve. In most individual species the numbers are practically equal. Thus, in the haddock, 48 have the left nerve uppermost and 52 the right nerve.

In the unsymmetrical teleosts or flounders, and soles, this condition no longer obtains. In those species of flounder with the eyes on the right side 236 individuals, representing sixteen species, had the left nerve uppermost in all cases.

Of flounders with the eyes on the left side, 131 individuals, representing nine species, all have the right nerve uppermost.

There are a few species of flounders in which reversed examples are so common that the species may be described as having the eyes on the right or left side indifferently. In all these species, however, whether dextral or sinistral, the relation of the nerves conforms to the type and is not influenced by the individual deviation. Thus the starry flounder (Platichthys) belongs to the dextral group. In 50 normal specimens, the eyes on the right have the left nerve dorsal, while the left nerve is also uppermost in 50 reversed examples with eyes on the left. In 15 examples of the California bastard halibut (Paralichthys californicus), normally sinistral, the right eye is always uppermost. It is uppermost in 11 reversed examples.

Among the soles this uniformity or monomorphism no longer obtains. In 49 individuals of four species of dextral soles, the left nerve is uppermost in 24, the right nerve in 25. Among sinistral soles, or tongue-fishes, in 18 individuals of two species, the left nerve is uppermost in 13, the right nerve in 5.

Professor Parker concludes from this evidence that soles are not degenerate flounders, but rather descended from primitive flounders which still retain the dimorphic condition as to the position of the optic nerves, a condition prevalent in all bony fishes except the flounders.

The lack of symmetry among the flounders lies, therefore, deeper than the matter of the migration of the eye. The asymmetry of the mouth is an independent trait, but, like the migration of the eye, is an adaptation to swimming on the side. Each of the various traits of asymmetry may appear independently of the others.

The development of the monomorphic arrangement in flounders Professor Parker thinks can be accounted for by the principle of natural selection. In a side-swimming fish the fixity of this trait has a mechanical advantage. The unmetamorphosed young of the flounder are not strictly symmetrical, for they possess the monomorphic position of the optic nerve. The reversed examples of various species of flounders (these, by the way, chiefly confined to the California fauna) afford "striking examples of discontinuous variation."

A very curious feature among the flounders is the possession in nine of the California-Alaskan species of an accessory half-lateral line. This is found in two different groups, while near relatives in other waters lack the character. One species in Japan has this trait, which is not found in any Atlantic species, or in any other flounders outside the fauna of northern California, Oregon, and Alaska.

Ancestry of Flounders.—The ancestry of the flounders is wholly uncertain. Because, like the codfishes, the flounders lack all fin-spines, they have been placed by some authors after the Anacanthini, or codfishes, and a common descent has been assumed. Some writers declare that the flounder is only a codfish with distorted cranium.

A little study of the osteology of the flounder shows that this supposition is without foundation. The flounders have thoracic ventrals, not jugular as in the cod. The tail is homocercal, ending in a large hypural plate, never isocercal, except in degraded soles, in which it is rather leptocercal. The shoulder-girdle, with its perforate hypercoracoid, has the normal perch-like form. The ventral fins have about six rays, as in the perch, although the first ray is never spinous. Pseudobranchiæ are developed, these structures being obsolete in the codfishes. The gills and pharyngeals are essentially as in the perch.

It is fairly certain that the Heterosomata have diverged from the early spiny-rayed forms, Zeoidei, Berycoidei, or Scombroidei of the Jurassic or Cretaceous, and that their origin is prior to the development of the great perch stock.

If one were to guess at the nearest relationships of the group, it would be to regard them as allies of the deep-bodied mackerel-like forms, as the Stromateidæ, or perhaps with extinct Berycoid forms, as Platycormus, having the ventral fins wider than in the mackerel. Still more plausible is the recent suggestion of Dr. Boulenger that the extinct genus Amphistium resembles the primitive flounder. But there is little direct proof of such relation, and the resemblance of larval flounders to the ribbon-fishes may have equal significance. But the ribbon-fishes themselves may be degenerate Scombroids. In any case both ribbon-fishes and flounders find their nearest living relatives among the Berycoidei or Zeoidei, and have no affinity whatever with the isocercal codfish or with other members of the group called Anacanthini.

The Heterosomata are found in all seas, always close to the bottom and swimming with a swift, undulatory motion. They are usually placed in a single family, but the degraded types known as soles may be regarded as forming a second family.

The Flounders: Pleuronectidæ.—In the flounders, or Pleuronectidæ, the membrane-bones of the head are distinct, the eyes large and well separated, the mouth not greatly contracted, and the jaws always provided with teeth. Among the 500 species of flounders is found the greatest variation in size, ranging in weight from an ounce to 500 pounds. The species found in arctic regions are most degenerate and these have the largest number of vertebræ and of fin-rays. The halibut has 50 vertebræ (16 + 34), the craig-flounder 58, while in Etropus and other tropical forms the number is but 34 (10 + 24). The common flounders of intermediate geographical range (Paralichthys dentatus, etc.) show intermediate numbers as 40 (10 + 30). The apparent significance of this peculiar series of fact is given on page 212, Vol. I. It is, perhaps, related to the greater pressure of natural selection in the tropics, showing itself in the better differentiation of the bones and consequently smaller number of the vertebræ.

Fossil flounders are very few and give no clue as to the origin of the group. In the Eocene and Miocene are remains which have been referred to Bothus (Rhombus). Bothus minimus is the oldest species known, described by Agassiz from the Eocene of Monte Bolca. In the Miocene are numerous other species of Bothus, as also tubercles referable to Scophthalmus.

On the testimony of fossils alone the genus Bothus, or one of its allies, would be the most primitive of the group. If it be so, the simpler structure of the halibut and its relatives is due to degeneration, which is probable, although their structure has the suggestion of primitive simplicity, especially in the greater approach to symmetry in the head and the symmetry in the insertion of the ventral fins.

Soles have been found in the later Tertiary rocks. Solea kirchbergiana of the Miocene is not very different from species now extant in southern Europe. No remains referable to allies of the halibut or plaice are found in Tertiary rocks, and these relatively simple types must be regarded as of recent origin.

The Turbot Tribe: Bothinæ.—The turbot tribe have the mouth large, the eyes and color on the left side, and the ventral fins unlike, that of the left side being extended along the ridge of the abdomen. The species are found in the warm seas only. They are deeper in body than the halibut and plaice, and some of them are the smallest of all flounders. It is probable that these approach most nearly of existing flounders to the original ancestors of the group.

Perhaps the most primitive genus is Bothus, species of which genus are found in Italian Miocene. The European brill, Bothus rhombus, is a common fish of southern Europe, deep-bodied and covered with smooth scales.

Very similar but much smaller in size is the half translucent speckled flounder of our Atlantic coast (Lophopsetta maculata), popularly known as window-pane. This species is too small to have much value as food. Another species, similar to the brill in technical characters but very different in appearance, is the turbot, Scophthalmus maximus, of Europe. This large flounder has a very broad body, scaleless but covered with warty tubercles. It reaches a weight of seventy pounds and has a high value as a food-fish. There is but one species of turbot and it is found in Europe only, on sandy bottoms from Norway to Italy. In a turbot of twenty-three pounds weight Buckland found a roe of five pounds nine ounces, with 14,311,260 eggs. The young retains its symmetrical condition for a relatively long period. No true turbot is found in America and none in the Pacific. Other European flounders allied to the turbot and brill are Zeugopterus punctatus; the European whiff, Lepidorhombus whiff-jagonis; the topknot, Phrynorhombus regius; the lantern-flounder, Arnoglossus laterna, and the tongue-fish, Eucitharus linguatula, the last two of small size and feeble flesh.

In the wide-eyed or peacock flounders, Platophrys podas in Europe, Platophrys lunatus, etc., in America, Platophrys mancus in Polynesia, the eyes in the old males are very far apart, and the changes due to age and sex are greater than in any other genera. The species of this group are highly variegated and lie on the sand in the tropical seas. Numerous small species allied to these abound in the West Indies, known in a general way as whiffs. The most widely distributed of these are Citharichthys spilopterus of the West Indies, Citharichthys gilberti and Azevia panamensis of Panama, Orthopsetta sordida of California, and especially the common small-mouthed Etropus crossotus found throughout tropical America. Numerous other genera and species of the turbot tribe are found on the coasts of tropical Asia and Africa, most of them of small size and weak structure.

Samaris cristatus of Asia is the type of another tribe of flounders and the peculiar hook-jawed Oncopterus darwini of Patagonia represents still another tribe.

The Halibut Tribe: Hippoglossinæ.—In the great halibut tribe the mouth is large and the ventral fins symmetrical. The arctic and subarctic species have the eyes and color on the right. Those of the warmer regions (bastard halibut) have the eyes and color on the left. These grow progressively smaller in size to the southward, the mouth being smaller and more feebly armed in southern species.

The largest of the family, and the one commercially of far greatest importance, is the halibut (Hippoglossus hippoglossus). This species is found on both shores of both oceans, north of about the latitude of Paris, Boston, Cape Mendocino, and Matsushima Bay in Japan. Its preference is for off-shore banks of no great depth, and in very many localities it exists in great abundance, reaching a length of 6 to 8 feet and a weight of 600 pounds. It sometimes ranges well out to sea and enters deeper waters than the cod. The flesh is firm, white, and of good quality, although none of the flatfishes have much flavor, the muscles being mostly destitute of oil. Small halibut, called "chicken halibut," are highly esteemed.

Dr. Goode states that the "history of the halibut fishery has been a peculiar one. At the beginning of the present century these fishes were exceedingly abundant on George's Banks; since 1850 they have partially disappeared from this region, and the fishermen have since been following them to other banks, and since 1874 out into deeper and deeper water, and the fisheries are now carried on almost exclusively in the gullies between the off-shore banks and on the outer edges of the banks, in water 100 to 350 fathoms in depth.

"The halibut with its large mouth is naturally a voracious fish, and probably would disdain few objects in the way of fresh meat it would come across. It is said, however, to feed more especially upon crabs and mollusks in addition to fish. These fish 'they waylay lying upon the bottom, invisible by reason of their flat bodies, colored to correspond to the general color of the sand or mud upon which they rest. When in pursuit of their prey they are active and often come quite to the surface, especially when in summer they follow the capelin to the shoal water near the land. They feed upon skates, cod, haddock, menhaden, mackerel, herring, lobsters, flounders, sculpins, grenadiers, turbot, Norway haddock, bank-clams, and anything else that is eatable and can be found in the same waters.' Frequently halibut may be seen chasing flatfish over the bottom of the water. About Cape Sable their favorite food seems to be haddock and cusk. A very singular mode of attacking a cod has been recorded by Captain Collins, an experienced fisherman and good observer. They often kill their prey by blows of the tail, a fact which is quite novel and interesting. He has described an instance which occurred on a voyage home from Sable Island in 1877: 'The man at the wheel sang out that he saw a halibut flapping its tail about a quarter of a mile off our starboard quarter. I looked through the spy-glass and his statement was soon verified by the second appearance of the tail. We hove out a dory, and two men went with her, taking with them a pair of gaff-hooks. They soon returned, bringing not only the halibut, which was a fine one of about seventy pounds weight, but a small codfish which it had been trying to kill by striking it with its tail. The codfish was quite exhausted by the repeated blows and did not attempt to escape after its enemy had been captured. The halibut was so completely engaged in the pursuit of the codfish that it paid no attention to the dory and was easily captured.'

"The females become heavy with roe near the middle of the year, and about July and August are ready to spawn, although 'some fishermen say that they spawn at Christmas' or 'in the month of January, when they are on the shoals.' The roe of a large halibut which weighed 356 pounds weighed 44 pounds, and indeed the 'ovaries of a large fish are too heavy to be lifted by a man without considerable exertion, being often 2 feet or more in length.' A portion of the roe 'representing a fair average of the eggs, was weighed and found to contain 2185 eggs,' and the entire number would be 2,182,773."

Closely allied to the halibut are numerous smaller forms with more elongate body. The Greenland halibut, Reinhardtius hippoglossoides, and the closely related species in Japan, Reinhardtius matsuuræ, differ from the halibut most obviously in the straight lateral line. The arrow-toothed halibut, Atheresthes stomias, lives in deeper waters in the North Pacific. Its flesh is soft, the mouth very large, armed with arrow-shaped teeth. The head in this species is less distorted than in any of the others, the upper eye being on the edge of the disk in front of the dorsal fin. For this reason it has been supposed to be the most primitive of the living species, but these traits are doubtless elusive and a result of degeneration.

Eopsetta jordani is a smaller halibut-like fish, common on the coast of California, an excellent food-fish, with firm white flesh, sold in San Francisco restaurants under the very erroneous name of "English sole." Large numbers are dried by the Chinese for export to China. A similar species, Hippoglossoides platessoides, known as the "sand-dab," is common on both shores of the North Atlantic, and several related species are found in the North Pacific. Verasper variegatus of Japan is notable for its bright coloration, the lower side being largely orange-red.

In the bastard halibuts, Paralichthys, the eyes and color are on the left side. These much resemble the true halibut, but are smaller and inferior as food, besides differing in details of structure. The Monterey halibut (Paralichthys californicus) is the largest of these, reaching a weight of sixty pounds. This species and one other from California (Xystreurys liolepis), normally left-sided, differ from all the other flounders in having the eyes almost as often on the right side as on the left side, as usual or normal in their type. The summer flounder (Paralichthys dentatus) replaces the Monterey halibut on the Atlantic Coast, where it is a common food-fish. Farther south it gives way to the Southern flounder (Paralichthys lethostigma) and the Gulf flounder, Paralichthys albigutta. In Japan Paralichthys olivaceus is equally common, and in western Mexico Paralichthys sinaloæ. The four-spotted flounder of New England, Paralichthys oblongus, belongs to this group. Similar species constituting the genus Pseudorhombus abound in India and Japan.

The Plaice Tribe: Pleuronectinæ.—The plaice tribe pass gradually into the halibut tribe, from which they differ in the small mouth, in which the blunt teeth are mostly on the blind side. The eyes are on the right side, the vertebræ are numerous, and the species live only in the cold seas, none being found in the tropics. In most of the Pacific species the lateral line has an accessory branch along the dorsal fin. The genus Pleuronichthys, or frog-flounders, has the teeth in bands. Pleuronichthys cornutus is common in Japan and three species, Pleuronichthys cœnosus being the most abundant, are found on the coast of California. Closely related to these is the diamond-flounder, Hypsopsetta guttulata of California. Parophrys vetulus is a small flounder of California, so abundant as to have considerable economic value. Lepidopsetta bilineata, larger and rougher, is almost equally common. It is similar to the mud-dab (Limanda limanda) of northern Europe and the rusty-dab (Limanda ferruginea) of New England.

The plaice, Pleuronectes platessa, is the best known of the European species of this type, being common in most parts of Europe and valued as food. Closely related to the plaice is a second species of southern Europe also of small size, Flesus flesus, to which the name flounder is in England especially applied. The common winter flounder of New England, Pseudopleuronectes americanus, is also very much like the plaice, but with more uniform scales. It is an important food-fish, the most abundant of the family about Cape Cod. The eel-back flounder, Liopsetta putnami, also of New England, is frequently seen in the markets. The males of this species have scattered rough scales, while the females are smooth. The great starry flounder of Alaska, Platichthys stellatus, is the largest of the small-mouthed flounders and in its region the most abundant. On the Pacific coast from Monterey to Alaska and across to northern Japan it constitutes half the catch of flounders. The body is covered with rough scattered scales, the fins are barred with black. It reaches a weight of twenty pounds. Living in shallow waters, it ascends all the larger rivers.

An allied species in Japan is Kareius bicoloratus, with scattered scales. Clidoderma asperrimum, also of northern Japan, has the body covered with series of warts.

In deeper water are found the elongate forms known as smear-dab and flukes. The smear-dab of Europe (Microstomus kitt) is rather common in deep water. Its skin is very slimy, but the flesh is excellent. The same is true of the slippery sole, Microstomus pacificus, of California and Alaska, and of other species found in Japan. Glyptocephalus cynoglossus, the craig-fluke, or pole-flounder, of the North Atlantic, is taken in great numbers in rather deep water on both coasts. Its flesh is much like that of the sole. A similar species (Glyptocephalus zachirus) with a very long pectoral on the right scale is found in California, and Microstomus kitaharæ in Japan.

The Soles: Soleidæ.—The soles (Soleidæ) are degraded flounders, the typical forms bearing a close relation to the plaice tribe, from which they may be derived. There are three very different groups or tribes of soles, and some writers have thought that these are independently derived from different groups of flounders. This fact has been urged as an argument against the recognition of the Soleidæ as a family separate from the flounders. If clearly proved, the soles should either be joined with the flounders in one family or else they should be divided into two or three, according to their supposed origin.

The soles as a whole differ from the flounders in having the bones of the head obscurely outlined, their edges covered by scales. The gill-openings are much reduced, the eyes small and close together, the ventral fins often much reduced, and sometimes the pectoral or caudal also. The mouth is very small, much twisted, and with few teeth.

The species of sole, about 150 in number, abound on sandy bottoms in the warm seas along the continents, very few being found about the Oceanic Islands. The three subfamilies, or tribes, may be designated as broad soles, true soles, and tongue-fishes.

The Broad Soles: Achirinæ.—The American soles (Achirinæ), or broad soles, resemble the smaller members of the turbot tribe of flounders, having the ventral fin of the eyed side extended along the ridge of the abdomen. The eyes and color are, however, on the right side. The eyes are separated by a narrow interorbital ridge. In most of these forms the body is broad and covered with rough scales. The species are mostly less than six inches long, and nearly all are confined to the warmer parts of America, many of them ascending the rivers. A very few (Aseraggodes, Pardachirus) are found in Japan and China. Some are scaleless and some have but a single small gill-opening on the blind side. The principal genus is Achirus. Achirus fasciatus, the common American sole, or hog-choker, is abundant from Boston to Galveston. Achirus lineatus and other species are found in the West Indies and on the west coast of Mexico. Almost all the species of Achirus are banded with black and the pectorals are very small or wanting altogether. All these species are practically useless as food from their very small size.

The European Soles (Soleinæ).—The European soles are more elongate in form, with the ventral fins narrow and not extended along the ridge of the abdomen. The eyes are on the right side with no bony ridge between them. No species of this type is certainly known from American waters, although numerous in Europe and Asia. The species have much in common with the plaice tribe of flounders and may be derived from the same stock. One species, as above noted, is found in the Miocene.

The common sole of Europe, Solea solea, is one of the best of food-fishes, reaching a length, according to Dr. Gill, of twenty-six inches and a weight of nine pounds. As usually seen in the markets it rarely exceeds a pound. It is found from Norway to Italy, and when properly cooked is very tender and delicate, superior to any of the flounders. According to Dr. Francis Day, it appears to prefer sandy or gravelly shores, but is rather uncertain in its migrations, for, although mostly appearing at certain spots almost at a given time, and usually decreasing in numbers by degrees, in other seasons they disappear at once, as suddenly as they arrive. Along the British seacoast they retire to the deep as frosts set in, revisiting the shallows about May if the weather is warm, their migrations being influenced by temperature. The food of the sole is to a considerable extent molluscous, but it is also said to eat the eggs and fry of other fishes and sea-urchins.

The spawning season is late in the year and during the spring months. The ova are in moderate number; a sole of one pound weight has, according to Buckland, about 134,000 eggs. The newly hatched, according to Dr. Day, do not appear to be commonly found so far out at sea as some other species. They enter into shallow water at the edge of the tide and are very numerous in favorable localities.

As is well known, the sole is one of the most esteemed of European fishes. In the words of Dr. Day, "the flesh of this fish is white, firm, and of excellent flavor, those from the deepest waters being generally preferred. Those on the west coast and to the south are larger, as a rule, than those towards the north of the British islands. In addition to its use as food, it is available for another purpose. The skin is used for fining coffee, being a good substitute for isinglass, and also as a material for artificial baits.

"The markets are generally supplied by the trawl. The principal English trawling-ground lies from Dover to Devonshire. They may be taken by spillers, but are not commonly captured with hooks; it is suggested that one reason may be that spillers are mostly used by day, whereas the sole is a night feeder. They are sometimes angled for with the hook, baited with crabs, worms, or mollusks; the most favorable time for fishing is at night, after a blow, when the water is thick, while a land breeze answers better than a sea breeze."

Several smaller species of sole are found in Europe. In Japan Zebrias zebra, black-banded, and Usinosita japonica, known as Usinóshita, or cow's tongue, are common. Farther south are numerous species of Synaptura and other genera peculiar to the Indian and Australian regions.

The Tongue-fishes: Cynoglossinæ.—The tongue-fishes are soles having the eyes on the left side not separated by a bony ridge, the two being very small and apparently in the same socket. The body is lanceolate, covered usually with rough scales, and as often with two or three lateral lines as with one. The species are mostly Asiatic. Cynoglossus robustus and other species are found in Japan, and in India are many others belonging to Cynoglossus and related genera. The larger species are valued as food. The single European species Symphurus nigrescens, common in the Mediterranean, is too small to have any value. Symphurus plagiusa, the tongue-fish of our coast, is common on our sandy shores from Cape Hatteras southward. Symphurus plagusia, scarcely different, replaces it in the West Indies. Symphurus atricandus is found in San Diego Bay, and numerous other species of no economic importance find their place farther south.