represent the clavicle. The humerus is usually wanting, while the radius and ulna are connected to the coracoid; to these succeed a variable number of bones supposed to represent the carpus, and the carpus, in turn, is followed by the fin rays, which are believed to be homologous to the metacarpal bones and phalanges. In the flying fish the pectoral fins are enormously developed; the hind limbs, called the ventral fins, are often absent, and sometimes, as in the cod, are placed in front of the pectoral. In those fins there are no representatives of the femur, tibia, fibula, or tarsus ; the "rays" unite directly with the pelvic arch, which is generally imbedded in the abdominal muscles, and is composed of only two bones supposed to represent the ischia. The dorsal and anal fins present nothing peculiar. The caudal fin, commonly called the tail, is always placed vertically, and is the chief motive power in progression; it is either homo-cercal (óμós, similar; and кéρкos, a tail) or hetero-cercal (repos, different; and кéρкos); the former most frequently appears in existing fishes, and consists of two equal lobes, while the vertebral column stops short at its base; the latter has the vertebral column prolonged into its upper lobe, so that it is unequally lobed. In the majority of fishes there exists a linear row of scales on each side of the body termed the "lateral line;" every scale in this line is perforated by a small duct opening into a longitudinal canal that runs along the entire length of the body, and communicates with certain cavities in the head. The use of this system is not known, for some state that it secretes mucus with which the body is covered, while others say that it is in some way connected with the sense of touch.

OSTEOLOGY.-The endoskeleton in most of the recent fishes is entirely converted into bone; in some it is partly bony and partly cartilaginous; in others it is wholly cartilaginous; while in the amphioxus the noto-chord remains

persistent throughout the life of the animal. The cartilaginous fishes have a very small amount of earthy matter in their skeleton, so that their bones hardly deserve the name; they are extremely elastic and flexible, and, in chemical composition, are said to resemble inspissated mucus. The bones of osseous fishes are much harder than the preceding, but are more flexible than the bones of the other classes of Vertebrata; they have no medullary canals, a small amount of spongy tissue, and are never laminated in the arrangement of their component parts. In most fishes the plasmatic canals are free from partial dilatations-garpike (belone) and sea-bream (sargus) form exceptions, The vertebral

column is divided into only two regions—an abdominal and a caudal; the vertebral centra are amphicælous, except in the bony pike (lepidosteus), in which they are opisthocælous; the margins of the contiguous vertebral centra are connected together by ligaments, and the cavities thus formed are filled with the gelatinous remains of the noto-chord, hence the extreme mobility of the entire spinal column. The abdominal vertebræ have a neural arch, a neural spine, and two transverse processes; the caudal vertebræ have, in addition to the neural arch, a hæmal arch and a hæmal spine, but no transverse processes. The ribs are imbedded in the muscles that surround the abdomen, and are connected to the transverse processes or bodies of the abdominal vertebræ at a single point; each rib generally gives off a styliform process of bone, which proceeds backwards amongst the muscles; there is no sternum. There is a series of dagger-like bones, called "interspinous bones," placed in the mesial line of the body between the lateral muscles; their internal points are generally situated between the spinous processes of the vertebræ, to which they are connected by ligament, while the fin-rays are joined to their opposite extremities by a peculiar joint, which will presently be described. Usually there is only one interspinous bone connected to the spinous process

of each vertebra, but in the flat-fishes (pleuro-nectidæ) there are two. There are two kinds of fin-rays, viz., the spinous rays and soft rays-hence fishes have been divided into acanthopterygii, those having spinous rays in their dorsal fins (ăкavea, a spine; and Teρúyιov, a little fin), and malacopterygii, in which all the rays are soft (μaλakóv, soft; and Tтeрúуiov). Every fin-ray is composed of two lateral pieces placed side by side; these are firmly united in the spinous rays, so as to simulate one bone, whereas in the soft rays they can be easily separated; each fin-ray articulates with its corresponding interspinous bone in a very peculiar manner, for the two pieces separate, so as to embrace the head of each interspinous bone, and end in blunt tubercles, which fit into depressions on each side of the interspinous bone; sometimes the interspinous bone is completely pierced, and the two pieces of the fin-ray meet, thus resembling the mode of union between the two links of a chain. The osteology of the skull of fishes is so intricate, and the opinions expressed by different authors concerning the homologies of the numerous bones forming the skull are so conflicting, that it is not my intention to discuss them in an elementary treatise such as this, but would refer the reader, desirous of information on this subject, to some of our larger standard works. The branchial apparatus shall, however, be briefly described in its proper place.

DIGESTION. The mouth of fishes is generally furnished with a complicated series of teeth, which, in the bony fishes, are generally firmly* anchylosed to the bones that support them, and are not only developed on the jaw-bones, but also on the other bones which surround the buccal cavity. Salivary glands are entirely wanting; the esophagus is

* The rostral teeth of the saw-fish are implanted in distinct sockets, as well as the teeth of a few others.

large and short, terminating in a well-defined stomach. The pyloric opening of the stomach is usually guarded by a valve; the intestinal canal is a simple tube commencing at the pyloric extremity of the stomach, and terminating at the anus or 66 vent." In the cartilaginous fishes the mucous membrane forms a spiral valve, winding like a screw from the pylorus to the anus, for the purpose of increasing the absorbent surface of the gut. The liver is generally large, soft, and saturated with oil; it occupies a large portion of the abdominal cavity, insinuating itself between the folds of intestine; a gall bladder is usually present. The pancreas in most fishes is, represented by a number of "cæcal tubes," called the "pyloric cæca," or pyloric appendages; it is, however, present in the shark, ray, pike, eel, while in the sturgeon it exists along with the pyloric cæca, and in the lepidosiren both pancreas and pyloric cæca are wanting. The kidneys are usually very large, extending the whole length of the abdominal cavity; they are placed close to the spine behind the peritoneum, and are composed of minute uriniferous tubules, variously contorted, which terminate in the ureters; a species of bladder, opening behind the anus, is frequently formed by the ureters. Most fishes possess a peculiar organ called the "swimming bladder," "* which is supposed to be homologous to the lungs of the higher vertebrata, but not analogous. The swimming bladder is a sac containing gas, situated beneath the spine, to which it is bound by the peritoneum. The shape of this organ varies somewhat in different fishes; in most it forms an elongated sac; in many it is divided by septa; while in the lepidosiren it is composed of two sacs, divided into a number of cellular spaces, and in this fish is both homologous and analogous to the lungs of the air-breathing vertebrata. In many fishes the swimming bladder forms a closed sac; in others there is

* It is absent in the skate, flat-fishes, and mackerel.

a duct leading from it called the "ductus pneumaticus," which opens into the esophagus (in the herring it opens into the stomach), and is homologous to the trachea of other vertebrata. The gas contained in this organ is secreted by its lining membrane, and is, in most fresh-water fishes, composed principally of nitrogen, and in sea fishes, of oxygen. The use of the swimming bladder is to alter the specific gravity of the fish, and to permit it to rise and sink in the water. This is accomplished by the alternate expansion and condensation of the contained gas, in consequence of either the relaxation and compression exerted by the abdominal walls, or of a muscular apparatus provided for the purpose.

RESPIRATION. On each side of a bony fish, immediately behind the head, there is a cavity called the branchial chamber, containing the branchiæ or gills, by means of which a fish respires; the external aperture of this chamber is covered by a large flap, called the operculum, which is constantly opening and shutting, to allow the escape of the water used in respiration. The bony framework of this large flap is formed by a chain of four broad flat bones, which have received the following names-præoperculum, operculum, sub-operculum, and inter-operculum. In addition to the operculum, the branchial chamber is also closed by a membrane, called the branchiostegal membrane (Bpáɣxia, gills; and σréyn, a cover), which is attached to the hyoid bone, and supported by a number of slender spines connected to the sides of the hyoid bone, termed "branchiostegal rays." The branchia, or gills, are composed of delicate leaflets, covered with a vascular mucous membrane, and are attached to the outer surface of a series of bones, termed the "branchial arches; these arches are usually four in number, and are connected with the hyoid bone below and the base of the skull above; their internal surfaces are generally studded with a series of processes which