prevent the food taken in by the mouth finding its way among the branchiæ, and thus interfering with the respiratory function. Respiration is accomplished in the following manner:-Water continually enters the mouth by a process analogous to swallowing, and then passes into the branchial chambers through a series of slits called branchial fissures, situated on both sides of the pharynx; the blood circulating through the branchial fringes robs of its oxygen the water, which finally escapes by the "gill slits" or "opercular aperture." CIRCULATION.—' -The heart* of fishes is a branchial or respiratory heart only, and consists of two cavities-an auricle and a ventricle. It is enclosed in a pericardium, and placed beneath the branchial apparatus. The blood takes the following course :-All the venous blood, coming both from the liver and body generally, is poured into the vena cava, which dilates into a large sinus before opening into the auricle; the auricular-caval opening is guarded by two membranous valves, which prevent regurgitation during the contraction of the auricle. The blood now passes into the ventricle through the auricular-ventricular opening, which is guarded by a strong mitral valve; from the ventricle arises a single artery called the branchial (also termed the right aortic arch); at its commencement this artery is dilated into a large sinus called the "bulbus arteriosus," which is separated from the ventricle by strong valves (in the shark tribe there are several rows of semilunar valves); the blood is propelled from the ventricle into the bulbus arteriosus, and thence through the branchial artery to the branchiæ, where it is purified; the radicles of the branchial * The heart is absent in the amphioxus, while that of the lepidosiren has three chambers. G veins take up the blood now purified, and these veins, coming from all the branchial arches, ultimately coalesce to form the aorta, which distributes it to all parts of the body. The aorta runs beneath the spine in the abdomen, giving off the usual visceral branches, but on reaching the caudal vertebræ it enters the hæmal arch, through which it passes to its termination. It is remarkable that, while all the muscles of a fish are pale, or even white, the muscular fibres of the heart are red. The action of the heart is slow, averaging from twenty to thirty beats per minute, and in many fishes its irritability continues for a long time after death. THE NERVOUS SYSTEM AND SENSES.-In fishes the nervous system presents a remarkable inferiority to that of the majority of vertebrate animals. The cavity of the cranium is small, and only partially occupied by the brain which consists of a collection of nervous ganglia, the first pair termed the olfactory lobes, the second pair the cerebral hemispheres, the third pair the optic lobes, and, lastly, a single ganglion resting on the medulla oblongata, and called the cerebellum. In addition to these, there are often present supplementary ganglia placed behind the cerebellum, supposed by some to be the anterior ganglia of the spinal cord greatly developed. All the cerebral nerves are present, with the exception of the ninth. The spinal nerves arise by double roots, the posterior roots, as in other vertebrata, being dilated into ganglia soon after their origin. The sympathetic system occupies the usual position, and communicates with the spinal nerves, but is small in comparison with that of other vertebrata. The olfactory nerves arising from the olfactory lobes vary in size and form in different fishes, nevertheless their destination is the same, viz., the olfactory chambers, which consist of two excavations situated near the tip of the snout, lined with a thin, plicated, pituitary membrane, behind which the nerve swells into a ganglion, from which numerous filaments proceed to be distributed on the surface of this membrane. Into the cavity of the nose, which is usually divided by a septum into two chambers, the water passes freely, generally by a double aperture, sometimes by a single one; these nasal chambers form closed sacs, and never communicate with the pharynx, except in the lepidosiren and myxine, or hag-fish. The optic nerves arise from the optic lobes, and are composed of broad bands of nervous substance, folded up like a fan, and enclosed in a dense membrane; they decussate completely, forming no commissure, and finally terminate in the retina. The eye of a fish possesses tunics and dioptric media the same as in man, but modified to suit the different condition under which its possessor is placed thus, the eye-ball is flattened from before backwards, and this shape is maintained by two cartilaginous plates imbedded in the sclerotic, while in some of the large fishes the sclerotic is converted into a cup of bone; the crystalline lens is spherical, and extremely dense, for the purpose of increasing the refraction of the rays of light, and is approximated to the retina at the expense of the vitreous humour. In many fishes the vitreous humour and lens are kept in position by a ligament called the falciform membrane, which passes from the retina through the vitreous humour, to be attached to the capsule of the lens. Behind the choroid coat there is placed an erectile organ called the "choroid gland," which is supposed to be of some use in arranging the focal distance of the lens. The pupil is large, but in the ray it is covered with a curtain, while in the anableps it is divided into two by a vertical septum forming a kind of natural stereoscope. The eye-ball is moved by six muscles, which have the same origin, distribution, and nervous supply as in man, except that the superior oblique does not pass through a pulley. The lachrymal glands and the eyelids are always wanting. The organ of hearing in fishes is very simple, and consists only of three semi-circular canals and a membranous vestibule, containing a glairy fluid and certain bodies of a stony hardness called otoliths (in cartilaginous fishes the otoliths, or ear stones, are soft). The external ear, membrana tympani, tympanic cavity, ossicles, Eustachian tube, and cochlea, are entirely wanting. The sense of taste is believed to be extremely imperfect, if at all present, while that of touch is located chiefly in the soft parts surrounding the mouth. The reproductive organs of osseous fishes present some remarkable points of interest. The ovary of the female, commonly called the roe, consists of a membranous bag, which at the spawning season becomes distended with ova, and occupies a considerable portion of the abdominal cavity. The ova, when mature, escape into the interior of the ovary, and are expelled through the oviduct, which opens either into the termination of the intestine, or by a distinct aperture situated behind the anus, and in front of the urinary canal. The testicle of the male, commonly called the "milt," or soft roe, consists of a multitude of seminiferous tubules, variously arranged in different fishes, but all terminating in a common duct. At the spawning season this organ becomes distended with a creamy fluid which is expelled into the water, and there vivifies the ova-thus, as a rule, the ova of osseous fishes are fecundated externally, but in a few, as, for example, the viviparous blenny, impregnation takes place internally. In the cartilaginous fishes, as well as in all other vertebrata, the ova burst through the capsule of the ovary, and enter the fallopian tubes, whence they are either directly expelled, or are retained and hatched internally. In sharks, rays, and dog-fishes, the ovum is protected by a horny case, which is secreted in the central portion of the oviduct; this case has four tendril-like cords attached to either corner, by means of which it is entangled in the sea-weed, and one of its extremities is open for the escape of the young animal. These cases are well known to |