ICHTHYOLOGY is that department of natural history which treats of Fist' ES, classifying them according to their forms, ascertaining their haunts, describing their manners, and specifying the uses to which they may be applied. The title of the science is a compound of two Greek words, a fish, and Aoyoc, a discourse.
Fishes belong to that great division of animals, charac terised by having an articulated skeleton, a bony cavity con taining the brain, a spinal marrow, and red blood. They are thus associated with quadrupeds, birds and reptiles, from which, however, they may be distinguished by the fol lowing characters.
Fishes possess a heart with one auricle and one ventricle. The blood is cold. They are destitute of a windpipe and lungs, breathing by means of peculiar organs termed gills. They reside in water, are in general covered with scales, and swim by the assistance of fins.
The ancients regarded as fishes, all animals which seek their food in water, or which reside in that element. Hence we find whales included by older writers among fishes, con stituting a subdivision, distinguished by their breathing by means of lungs. But cetaceous animals differ from fishes, not merely in their organs of respiration, but likewise in those of circulation and reproduction. Their heart consists of two auricles and two ventricles, and the blood is warm. They Wing forth their young alive, and suckle them.
It is more difficult to draw the line of distinction between fishes and reptiles. The structure of the heart, and the temperature of the blood, are similar in both classes. Rep tiles, however, are all furnished with lungs ; and although, in a few genera, the young are possessed of gills, yet these organs disappear before the animals arrive at maturity. There are, however, two genera, in which the gills are per sistent, but the animals belonging to these have digitated feet, and are not likely ever to be confounded with fishes.
The study of this department of natural history is pecu liarly difficult. The element in which fishes reside con ceals their movements from our view, and prevents us from becoming acquainted with their instincts and habits. In the midst of lakes and rivers, in the depths of the ocean, or in pools on the less frequented shores, they perform unmo lested the great operations of nature. Hence the manner
of their production, the appearances exhibited by the dif ferent sexes, the changes produced by age or season, the artifices they employ to obtain their food, the migrations which they perform, and the limits of their existence, have hitherto been but imperfectly ascertained.
The rarer kinds of fishes, occasionally found on the hooks or in the nets of fishers, are rejected by them as useless, or perhaps exhibited for a few days in the neighbouring vil lage as objects of wonder. They speedily decay, and the only memorials of their existence are preserved in the im perfect traditions of their form. It rat ely happens that an intelligent naturalist is on the spot to examine their cha racters, or to delineate their appearance: But even should there be a person at hand capable of forming a systematic description, still their haunts and habits must rernaiiv un known to us. Of the various species of fishes enumerated by nomenclaturists, pet haps more than the one half are known to us only by their external forms ; their food, their instincts, and their functions, remaining unascertained.
The pursuits of the ichthyologist are far from possessing those charms which excite the zeal and admiration of the naturalist, in other departments of zoology. It is easy, in deed, to form a collection of these objects, and to preserve their form and characters for scientific purposes; but a museum furnished with such specimeits presents little that is gay or splendid. The colours and lustre of fish fade after death, nor can any process of embalming fix and preserve them. A coating of varnish is often resorted to as a substitute for the natural slimy covering of their skins, and the brush is emplo)ed to restore the colours ; but speci mens prepared in this manner are disregarded by the scien tific observer, and, to the mere spectator, a collection of fishes in wax-work would be equally acceptable. The as pect of some fishes, as the eel for example, is repulsive ; whilst the cold moist surface of others, disgust the fastidi ous naturalist, or at least lessen his anxiety to examine this portion of animated nature.