When Pliny (23-79 a.d.) wrote his Historia Naturalis, he enumerated 94 kinds of fishes then known to the Roman world. In 1735 Linnaeus listed 478. Today it is estimated that at least 25,000 species are known.
The importance of this class of vertebrates is brought out by the fact that the waters of the earth, of which fishes are the dominant inhabitants, occupy four times the area of the land masses combined. Although primeval fishes lived in fresh water before the oceans became salty, only approximately one species out of twelve now lives in inland fresh waters, while ten are distributed in open oceans or coastal regions and one is a dweller in the darkness of the deep sea.
Fishes vary enormously in size all the way from the dainty Mistichthys luzonensis of the Philippines, which is about half an inch long when full grown, to the colossal shark, Rhinodon typicum that, according to Haempel, has been known to attain a length of 65 feet. There is likewise a remarkable range in the body form of fishes, as indicated by representatives shown in outline in Fig. 18.
The key for understanding the fish plan is to be sought in the adaptation of these animals to life in water. For example, their chief respiratory organs are internal gills. The tail, which frequently is more extensive than all the rest of the body, is the main organ of propulsion, entirely effective in sculling through the resistant medium of water, but quite useless in thin air. The other unpaired fins as well as the paired ones, which are homologous with our arms and legs, are used for steering and to prevent rotation of the body on its long axis, which would otherwise occur as the result of the sweeping strokes of the tail.
As in all higher vertebrates, skeletal rings, the beginnings of the centra of vertebrae, encircle the embryonic notochord of most fishes. During development, these rings gradually fill in to constrict the enclosed notochord considerably in fishes, and obliterate it almost completely in higher forms. Well-developed skulls, consisting of elements associated with both brain and gills, first occur in fishes. In some of the lowest fishes (Cladoselache, for example) it can be seen that biting jaws arise as modifications of the most anterior of the gill supports. Because of the presence of jaws, fishes and all higher vertebrates may be grouped together as Gnathostomata (gnath, jaw; stoma, mouth).
As far as living fishes are concerned, the Pisces may be divided into two subclasses:
a. Chondrichthyes (chondro, cartilage; ichthy, fish)
b. Osteichthyes (osteo, bone)
The cartilaginous fishes are so called because their skeletons are entirely of cartilage, the material commonly called gristle. There is considerable evidence that the absence of bone from these animals may be a degenerate condition. Many believe that the ancestors of all the fishes were forms similar to the Ostracoderms.
None of the cartilaginous fishes has an air bladder. The intestinal surface available for both secretion and absorption is considerably increased by a spiral valve (Fig. 212), a tightly coiled fold of the inner part of the intestinal wall.
The Orders of cartilaginous fishes include: (1) Cladoselachii; (2) Elasmobranchii; and (3) Holocephali.
The long extinct, primitive sharks belonging to the genus Cladoselache possessed many interesting features (Fig. 19). The jaws of these ancient sharks were clearly in line with the gill arches which lay immediately behind them. In these animals it is even more obvious than in modern sharks that the jaws represent the first pair of gill arches. The ventral halves of the first arches have become the lower jaw while the dorsal halves, tilted forward, have become the upper jaw. A rudimentary vertebral column, consisting of an unconstricted notochord onto which were laced vertebral arches, was present.
The fins of Cladoselache had broad attachments to the body, a condition which may be cited in support of the fin-fold theory of origin of appendages. According to this theory the first appendages were a set of fin folds, including: (1) a continuous median fin extending along the dorsal side of the body, around the tail and forward along the ventral side of the body as far as the anus; (2) a pair of ventro-lateral folds extending from the anal region forward to points immediately behind the last gill openings. Individual fins may have been derived from the dropping out of portions of these folds and the retention of other portions. The primitive individual fins would therefore presumably have broad attachments, such as those of Cladoselache. Then a later development would be the narrowing of the base from which the fins fan out, as in many modem fishes.
Elasmobranchs (gristle fishes)
Most living cartilaginous fishes are elasmobranchs, including the sharks (Suborder Selachii) and the skates and rays (Suborder Batoidea). The sharks, including dogfishes and their allies (Fig. 20A and c), all graceful, elongated, streamlined animals, actively prey upon other fishes. The rays, with the skates, torpedoes, guitar fishes, and their allies, on the contrary, are flattened sluggish bottom feeders (Fig. 20B). In the Batoidea the pectoral fins, considerably enlarged, are the organs of propulsion.
An elasmobranch may be distinguished by the following characteristics: (1) a large mouth ventral in position rather than terminal; (2) separate openings of the gill slits (usually five pairs) not concealed as in other fishes behind a gill cover; (3) dorso-ventrally flattened head; (4) placoid scales, resembling tiny thumb tacks embedded, point up, in the skin without shingling over each other like ordinary fish scales; (5) tail heterocercal, that is, with the vertebral column extending into the dorsal part of the caudal fin; (6) paired pelvic fins modified into “clasping organs” in the male, permitting internal fertilization of the egg; and (7) the production of a relatively small number of eggs that in some species are covered with shells and then laid, while in others they develop into some size within the oviduct of the female before the young are bom alive.
Biological interest in the uncommon and bizarre “elephant fishes,” or “spook fishes,” centers in their intermediate anatomical position between elasmobranchs and other fishes. They differ from elasmobranchs in a number of respects among which are: (1) small size of mouth; (2) scarcity of scales; and (3) presence of a gill cover, or operculum, a fold of the body wall extending over the gill slits but with a free posterior margin under which the respiratory current leaves the body.
The name holocephali (holos, whole; cephalon, head) is given to them because the upper jaw is immovably fused with the cranium after the manner of higher forms, instead of being indirectly suspended by means of ligaments and cartilages as in elasmobranchs.
Of the three living genera, Chimaera (Fig. 21) is found on the Pacific coast of North America, as well as on the coasts of Europe and Japan and at the Cape of Good Hope.
The modern bony fishes are grouped into the subclass Osteichthyes. At least a part of the skeleton is ossified. Over the skull and pectoral girdle region, investing bony plates are laid down in the dermal portion of the skin. The gills are protected by an opercular fold which has investing bony supports. No claspers are present. Air bladders, outgrowths from the pharynx, are present in all bony fish except a few teleosts.
There are three orders of bony fishes: (1) Crossopterygii; (2) Dipnoi; and (3) Actinopterygii.
The Crossopterygii are commonly called lobe-finned fishes because each of their paired fins has a thick, fleshy basal lobe. The skeletal support of this fin consists of a single basal element followed by two bones beyond which are a number of irregular small bones. This condition suggests the plan of the land type of appendage. Other features of this group are: (1) air bladders serving as lungs; (2) a spiral valve in the intestine; and (3) nasal cavities opening into the roof of the mouth cavity. These fish are believed to be the ancestors of land animals.
All members of this group are extinct with the exception of Latimeria which is represented by one specimen, about 5 feet long, brought up in a net off the coast of South Africa in 1939. Unfortunately the internal parts of this specimen were destroyed before its zoological significance became known.
As the word dipnoi (di, two; pneum, air) suggests, the “lungfishes” have two ways of breathing, that is, by means of gills and by a modified swim bladder, or lung.
They are semitropical, freshwater, primitive fishes dwelling only in countries where wet and dry seasons alternate, instead of winter and summer. During the dry season the African and South American species bury themselves in muddy pits (Fig. 22) and breathe air like land animals, but when the rainy season supplies an abundance of water they swim about fish-fashion, breathing through gills. This passive method of bridging over a season of unfavorable dryness is termed aestivation, corresponding to hibernation, or the habit of animals like bears, bats, and woodchucks, which retire from activity during the cold winter season.
Other interesting features of these fishes are: (1) a spiral valve; (2) nasal passages opening into the roof of the mouth cavity; and (3) fleshy portions in the paired fins.
There are no lungfishes in North America, and only three living genera are known anywhere. These are Protopterus in Africa; Neoceratodus in Australia (Fig. 23); and Lepidosiren in Brazil (Fig. 24). They possess much interest for the zoologist not only by reason of their peculiar habits and rarity, but also because of the intermediate combination of their anatomical characters which puts them in a class by themselves.
Included among the ray-finned fishes (actin, ray; pteryg, fin) are the several groups possessing fins with no basal lobes but with their entire free parts membranous and supported by slender dermal rays. Their nasal pits do not communicate with the mouth cavity. This group includes 3 sub-orders: Chondrostei, Holostei and Teleostei.
Chondrosteans. Those ray-finned fishes which have superficial, dermal, bony plates but an inner skeleton composed essentially of cartilage are called Chondrostei (chondro, cartilage; osteo, bone). These animals also possess a spiral valve and ganoid scales. Among the living members of this group are Polypterus, Acipenser, Scaphirhynchus, and Polyodon (Fig. 25).
Polypterus (poly, many; pter, fin), so named because it has a row of small dorsal fins, is found in the tropical fresh waters of Africa. A well-developed pair of swim bladders is connected with the ventral side of the pharynx. A secondarily acquired basal lobe in the pectoral fins has led some taxonomists to include this animal among the Crossopterygii.
Acipenser and Scaphirhynchus are the sturgeons of the rivers and lakes of the Northern Hemisphere. Caviar, the delicacy made famous by the Russians, is the eggs of sturgeons. Like the sharks these animals have a rostrum, a ventral mouth, and a heterocercal tail. The scales are reduced in number. In Polyodon, the Mississippi spoon-bill, the rostrum is greatly enlarged into a dorso-ventrally flattened, paddle-like structure and the skin is almost devoid of scales. The sturgeons and spoon-bills have a persistent, unconstricted notochord onto which cartilaginous arches are laced by connective tissue.
Holosteans. Those ray-finned fishes which have superficial, dermal, bony plates and also a rather completely ossified inner replacing skeleton are called Holostei (holo, complete; osteo, bone). Other features are: (1) a reduced spiral valve; (2) bony vertebrae; and (3) ganoid scales. There are only two living genera of this group. Lepidosteus (Fig. 26A), the garpike of the fresh waters of North America, possesses heavy, shiny, ganoid scales which fit together edge to edge, rarely overlapping, like the tiles around a fireplace. Amia (Fig. 26B), the freshwater dogfish of the United States, has overlapping, cycloid scales on the trunk and tail regions, ganoid scales being limited to the head region.
The Chondrostei and Holostei are frequently referred to as the Ganoid fishes, with the former being called the cartilaginous ganoids and the latter, the bony ganoids.
Teleosts. The Teleostei (tele, entire; osteo, bone) are the true bony fishes. They constitute probably 90 per cent of all known kinds of fishes. They have an almost completely bony skeleton; no spiral valve; thin, round overlapping scales on most species; and a homocercal tail. A few unusual representatives out of the great variety of teleosts are suggested by outline sketches in Figure 18. Some of the best known teleosts are herring, salmon, trout, eels, catfish, mackerel, carps, pickerel, perch, flounders, and cod. Of the carps alone there are approximately 1000 species.