CHAPTER XI

ACANTHOPTERYGII; SYNENTOGNATHI

=ORDER Acanthopterygii, the Spiny-rayed Fishes.=—The most of the remaining bony fishes constitute a natural group for which the name _Acanthopterygii_ (ἄκανθα, spine; πτερύξ, πτερόν, fin or wing) may be used. This name is often written _Actinopteri_, a form equally correct and more euphonious and convenient. These fishes are characterized, with numerous exceptions, by the presence of fin spines, by the connection of the ventral fins with the shoulder-girdle, by the presence in general of more than one spine in the anterior part of dorsal and anal fins, and as a rule of one spine and five rays in the ventral fins, and by the absence in the adult of a duct to the air-bladder. Minor characters are these: the pectoral fins are inserted high on the shoulder-girdle, the scales are often ctenoid, and the edge of the upper jaw is formed by the premaxillary alone, the maxillary being always toothless.

But it is impossible to define or limit the group by any single character or group of characters. It is connected with the _Malacopterygii_ through the _Haplomi_ on the one hand by transitional groups of genera which may lack any one of these characters. On the other hand, in the extreme forms, each of these distinctive characters may be lost through degeneration. Thus fin spines, ctenoid scales, and the homocercal tail are lost in the codfishes, the connection of ventrals with shoulder-girdle fails in the _Percesoces_, etc., and the development of the air-duct is subject to all sorts of variations. In one family even the adipose fin remains through all the changes and modifications the species have undergone.

The various transitional forms between the _Haplomi_ and the perch-like fishes have been from time to time regarded as separate orders. Some of them are more related to the perch, others rather to ancestors of salmon or pike, while still others are degenerate offshoots, far enough from either.

On the whole, all these forms, medium, extreme and transitional, may well be placed in one order, which would include the primitive flying-fishes and mullets, the degraded globefishes, and the specialized flounders. As for the most part these are spiny-rayed fishes, Cuvier's name _Acanthopterygii_, or _Acanthopteri_, will serve us as well as any. The _Physoclysti_ of Müller, the _Thoracices_ of older authors, and the _Ctenoidei_ of Agassiz include substantially the same series of forms. The order _Teleocephali_ of Gill (τελεός, perfect; κεφαλή, head) has been lately so restricted as to cover nearly the same ground. In Gill's most recent catalogue of families, the order _Teleocephali_ includes the _Haplomi_ and rejects the _Hemibranchii_, _Lophobranchii_, _Plectognathi_, and _Pediculati_, all of these being groups characterized by sharply defined but comparatively recent characters not of the highest importance. As originally arranged, the order _Teleocephali_ included the soft-rayed fishes as well. From it the _Ostariophysi_ were first detached, and still later the _Isospondyli_ were regarded by Dr. Gill as a separate order.

We may first take up serially as suborders the principal groups which serve to effect the transition from soft-rayed to spiny-rayed fishes.

=Suborder Synentognathi.=—Among the transitional forms between the soft-rayed and the spiny-rayed fishes, one of the most important groups is that known as _Synentognathi_ (σύν, together; ἔν, within; γνάθος, jaw). These have, in brief, the fins and shoulder-girdle of _Haplomi_, the ventral fins abdominal, the dorsal and anal without spines. At the same time, as in the spiny-rayed fishes, the air-bladder is without duct and the pectoral fins are inserted high on the side of the body. With these traits are two others which characterize the group as a suborder. The lower pharyngeal bones are solidly united into one bone and the lateral line forms a raised ridge along the lower side of the body. These forms are structurally allied to the pikes (_Haplomi_), on the one hand, and to the mullets (_Percesoces_), on the other, and this relationship accords with their general appearance. In this group as in all the remaining families of fishes, there is no mesocoracoid, and in very nearly all of these families the duct to the air-bladder disappears at an early stage of development.

=The Garfishes: Belonidæ.=—There are two principal groups or families among the _Synentognathi_, the _Belonidæ_, with strong jaws and teeth, and the _Exocœtidæ_, in which these structures are feeble. Much more important characters appear in the anatomy. In the _Belonidæ_ the third upper pharyngeal is small, with few teeth, and the maxillary is firmly soldered to the premaxillary. The vertebræ are provided with zygapophyses. The species of _Belonidæ_ are known as garfishes, or needle-fishes. They resemble the garpike in form, but have nothing else in common. The body is long and slender, covered with small scales. Sharp, unequal teeth fill the long jaws and the dorsal is opposite the anal, on the hinder part of the body. These fishes are green in color, even the bones being often bright green, while the scales on the sides have a silvery luster. The species are excellent as food, the green color being associated with nothing deleterious. All are very voracious and some of the larger species, 5 or 6 feet long, may be dangerous even to man. Fishermen have been wounded or killed by the thrust of the sharp snout of a fish springing into the air. The garfishes swim near the surface of the water and often move with great swiftness, frequently leaping from the water. The genus _Belone_ is characterized by the presence of gill-rakers. _Belone belone_ is a small garfish common in southern Europe. _Belone platura_ occurs in Polynesia. The American species (_Tylosurus_) lack gill-rakers. _Tylosurus marinus_, the common garfish of the eastern United States, often ascends the rivers. _Tylosurus raphidoma_, _Tylosurus fodiator_, _Tylosurus acus_, and other species are very robust, with short strong jaws. _Athlennes hians_ is a very large fish with the body strongly compressed, almost ribbon-like. It is found in the West Indies and across the Isthmus as far as Hawaii. Many other species, mostly belonging to _Tylosurus_, abound in the warm seas of all regions. _Tylosurus ferox_ is the long tom of the Australian markets. _Potamorrhaphis_ with the dorsal fin low is found in Brazilian rivers. A few fossil species are referred to _Belone_, _Belone flava_ from the lower Eocene being the earliest.

=The Flying-fishes: Exocœtidæ.=—The family of _Exocœtidæ_ includes the flying-fishes and several related forms more or less intermediate between these and the garfishes. In these fishes the teeth are small and nearly equal and the maxillary is separate from the premaxillary. The third upper pharyngeal is much enlarged and there are no zygapophyses to the vertebræ.

The skippers (_Scombresox_) have slender bodies, pointed jaws, and, like the mackerel, a number of detached finlets behind dorsal and anal, although in other respects they show no affinity to the mackerel. The common skipper, or saury (_Scombresox saurus_), is found on both shores of the North Atlantic swimming in large schools at the surface of the water, frequently leaping for a little distance like the flying-fish. They are pursued by the mackerel-like fishes, as the tunny or bonito, and sometimes by porpoises. According to Mr. Couch, the skippers, when pursued, "mount to the surface in multitudes and crowd on each other as they press forward. When still more closely pursued, they spring to the height of several feet, leap over each other in singular confusion, and again sink beneath. Still further urged, they mount again and rush along the surface, by repeated starts, for more than one hundred feet, without once dipping beneath or scarcely seeming to touch the water. At last the pursuer springs after them, usually across their course, and again they all disappear together. Amidst such multitudes—for more than twenty thousand have been judged to be out of the water together—some must fall a prey to the enemy; but so many hunting in company, it must be long before the pursuers abandon. From inspection we could scarcely judge the fish to be capable of such flights, for the fins, though numerous, are small, and the pectoral far from large, though the angle of their articulation is well adapted to raise the fish by the direction of their motions to the surface."

A similar species, _Cololabis saira_, with the snout very much shorter than in the Atlantic skipper, is the _Samma_ of the fishermen of Japan.

The hard-head (_Chriodorus atherinoides_) has no beak at all and its tricuspid incisor teeth are fitted to feed on plants. In this genus, as in the flying-fishes, there are no finlets. The hard-head is an excellent food-fish abundant about the Florida Keys but not yet seen elsewhere.

Another group between the gars and the flying-fishes is that of the halfbeaks, or balaos, _Hemirhamphus_, etc. These are also vegetable feeders, but with much smaller teeth, and the lower jaw with a spear-like prolongation to which a bright-red membrane is usually attached. Of the halfbeaks there are several genera, all of the species swimming near the surface in schools and sometimes very swiftly. Some of them leap into the air and sail for a short distance like flying-fishes, with which group the halfbeaks are connected by easy gradations. The commonest species along our Atlantic coast is _Hyporhamphus unifasciatus_; a larger species, _Hemirhamphus brasiliensis_, abounds about the Florida Keys. _Euleptorhamphus longirostris_, a ribbon-shaped elongate fish, with long jaw and long pectorals, is taken in the open sea, both in the Atlantic and Pacific, being common in Hawaii. The Asiatic genus _Zenarchopterus_ is viviparous, having the anal fin much modified in the male, forming an intromittent organ, as in the _Pœciliidæ_. One species occurs in the river mouths in Samoa.

The flying-fishes have both jaws short, and at least the pectoral fins much enlarged, so that the fish may sail in the air for a longer or shorter distance.

The smaller species have usually shorter fins and approach more nearly to the halfbeaks. _Fodiator acutus_, with sharp jaws, and _Hemiexocœtus_, with a short beak on the lower jaw, are especially intermediate. The flight of the flying-fishes is described in detail on p. 157, Vol. I.

The Catalina flying-fish, _Cypselurus californicus_, of the shore of southern California is perhaps the largest of the known species, reaching a length of 18 inches. To this genus, _Cypselurus_, having a long dorsal and short anal, and with ventrals enlarged as well as pectorals, belong all the species strongest in flight, _Cypselurus heterurus_ and _furcatus_ of the Atlantic, _Cypselurus simus_ of Hawaii and _Cypselurus agoo_ in Japan. The very young of most of these species have a long barbel at the chin which is lost with age.

In the genus _Exonautes_ the base of anal fin is long, as long as that of the dorsal. The species of this group, also strong in flight, are widely distributed. Most of the European flying-fishes, as _Exonautes rondeleti_, _Exonautes speculiger_, and _Exonautes vinciguerræ_, belong to this group, while those of _Cypselurus_ mostly inhabit the Pacific. The large Australian species _Exonautes unicolor_, Fig. 226, Vol. I, belongs to this group. In the restricted genus _Exocœtus_ the ventral fins are short and not used in flight. _Exocœtus volitans_ (_evolans_) is a small flying-fish, with short ventral fins not used for flight. It is perhaps the most widely distributed of all, ranging through almost all warm seas. _Parexocœtus brachypterus_, still smaller, and with shorter, grasshopper-like wings, is also very widely distributed. An excellent account of the flying-fishes of the world has been given by Dr. C. F. Lütken (1876), the University of Copenhagen, which institution has received a remarkably fine series from trading-ships returning to that port. Later accounts have been given by Jordan and Meek, and by Jordan and Evermann.

Very few fossil _Exocœtidæ_ are found. Species of _Scombresox_ and _Hemirhamphus_ are found in the Tertiary, the earliest being _Hemirhamphus edwardsi_ from the Eocene of Monte Bolca. No fossil flying-fishes are known, and the genera, _Exocœtus_, _Exonautes_, and _Cypselurus_ are doubtless all of very recent origin.