Page:The American Cyclopædia (1879) Volume VII.djvu/246

 238 FISHES only prehensile and principal tactile organs, with the exception of the barbels and other appendages above alluded to. The corneous, slightly movable, and often tooth-armed tongue receives but few nerves, and cannot be the seat of any sense worthy of the name of taste ; and moreover, the food does not remain long enough in the mouth for any exercise of this sense. The olfactory apparatus is more com- plicated, but it is traversed neither by air nor the water used in respiration ; the nasal cavi- ties do not communicate with the mouth. The ear, almost always entirely within the cranium, on the sides of the brain, consists essentially of a vestibule and three semicircular canals, which receive the vibrations of the integuments and cranial walls ; there is rarely anything that can be called external ear, drum, or tympanic cavity ; loud, sudden, and strange sounds fright- en fishes, as the experience of every fisherman tells him ; in ancient, and even in modern times, they have been taught to come and re- ceive food at the tinkle of a bell, or the pro- nunciation of pet names. The eyes have neither true lids nor lachrymal apparatus ; the pupil is large and permanently open, the lens is spherical, and the flat cornea is covered by the skin. Fishes are very voracious, most of them living on animal food, and swallowing indiscriminately anything of this kind which comes in their way ; some genera, like the lamprey eels, live upon the juices of other fish, and the mouth is provided with circular car- tilages, fleshy disks, teeth, and a piston-like tongue, which enable them to adhere to any surface. The intestinal canal is short and sim- ple, and digestion is rapidly performed, and their increase in size is remarkably affected by the nature and abundance of their food ; their limits as to size and the natural duration of life are very little known in the great majority of species. The blood of fishes is red, and the globules are elliptical and of considerable size. The heart is placed under the throat in a cavity separated from the abdomen by a kind of dia- phragm, protected by the pharyngeal bones above, the branchial arches on the sides, and generally by the scapular arch behind; it con- sists of a venous sinus, auricle, ventricle, and bulb ; all these cavities circulate venous blood, and therefore physiologically correspond to the right side of the mammalian heart, though Owen says that the heart of fishes with the muscular branchial artery is the true homologue of the left auricle, ventricle, and aorta of higher vertebrates, tracing the complication of the organ synthetically ; the auricle and ven- tricle, however, are alone proper to the heart itself, the sinus being the termination of the venous system, and the bulb an addition to the pulmonary artery; these four compartments, therefore, are not like the four divisions of the human heart, but succeed each other in a linear series. The circulation is double, that of the system at large and that of the branchise being complete and distinct, and there is also an ab- dominal circulation terminating at the liver; the peculiar character is that the branchial circulation alone is provided with a propelling cavity or heart, the branchial veins changing into arteries without any intermediate left au- ricle and ventricle. The venous sinus receives the blood from the general system, after the manner of vense cavse ; it is not usually situ- ated within the pericardium. The auricle, when distended, is larger in proportion to the ventricle than in the higher vertebrates; its walls are membranous, with thin muscular fas- ciculi, and its simple cavity communicates with the ventricle by a single opening guarded by free semilunar valves, two to four in number. The ventricle, usually a four-sided pyramid, is very muscular, and its fibres are redder than those of any other part of the system ; its cav- ity is simple, the auricular valve generally free and without chordaa tendinese, and its opening into the bulb provided with two or four semi- lunar valves. The contractile lulbus arterio- sus is provided in the ganoids and plagiostomes with several rows of valves, and its muscular walls are distinct from those of the ventricle. The immediate force of the heart's action is applied through the continuation of the bulb into the branchial artery, which is generally short, and is divided into lateral branches going to the gills ; the blood, which has become ar- terialized by its subjection to the air contained in the respired water, is carried along the re- turning vessels into the branchial veins, the analogues of the pulmonary veins of man ; the four on each side form the aortic circle from which the pure blood is sent over the system through the carotids and the aorta and its branches ; the blood of the chylopoietic viscera passes through the liver before entering .the great sinus. Though all the blood passes through the branchial apparatus, it traverses the heart but once. Respiration is effected by means of the innumerable vascular lamellae and tufts attached to the external edge of the branchial arches; these are generally four on each side, each composed of two rows of fringes; inmost cartilaginous fishes there are five, and in the lamprey seven ; in the last fish there is a canal from the mouth to the respira- tory cavity, resembling a trachea. Fishes con- sume but a small amount of oxygen, but some, not content with that contained in the water, come to the surface occasionally to swallow air ; they perish soon out of water in propor- tion to the quickness with which the gills be- come dry, asphyxia being produced not by the want of oxygen dir,ectly, but because the blood cannot circulate in them properly unless sus- tained and kept soft by water. Though fishes produce little heat, some possess the singular faculty of generating and discharging electri- city. (See ELECTKIC FISHES.) Fishes reproduce by means of eggs, the number of which in some species amounts to hundreds of thousands; these have generally only a mucilaginous en- velope, and are fecundated after being laid;