ICHTHYOLOGY, is that part of natu ral treats of fishes. And fish . es are animals a heart with one auricle and one ventricle, with cold red blood, which inhabit water, and breathe I by means of gills. Most Of the species are likewise distinguished by fins and scales. Different naturalists have given different systems; we shall briefly notice some of the principal, because we find them perpetually referred to in works of importance. Aldrovandus, about two hun dred years ago, distributed the fishes ac cording to the nature of their residence. His first book treats of those that frequent rocks ; the second is devoted to those found near the shores, called littoral ; the third, pelagian, &c. Willoughby formed his system from his observation on the anatomy and physiology of fishes; he was followed by Ray, who fixed a series of genera. Artedi, the friend and country man of Linmeus, has the merit of having first traced the outlines of that classifica tion of fishes, which is now almost gene rally adopted. For, independently of the cetaceous tribes, which are now generally classed with the mammalia, his method consisted of four orders, viz. 1. The mal a coterygian, or those which have soft fins, or fins with bony rays, but without spines. 2. The acanthopterian, those with spiny fins. 3. The branchiostegous, corres ponding to the amphibia nantes of Lin nzus. 4. The chondropterygian, or those which have not true bones, but only car tilages, and the rays of whose fins hardly differ from a membrane. At first Linn mos adopted this method entirely, but he af terwards improved upon it ; and now, ac cording to his system, the orders have been instituted from the situation, pre sence, or absence of the ventral fins.
1. Such .as are entirely destitute of these fins are termed pisces epodes, apo dal or footless fishes. 2. The jugulares, or jugular, are those which have ventral fins, placed more forward than the pecto ral fins, or under the throat. 3. The tho racici, or thoracic, include those whose ventral fins are placed immediately under the pectoral fins, or on the breast. 4. The abdominales, or abdominal, comprise those whose ventral fins are situated be hind the pectoral fins, or on the abdomen. 4. There still remains a tribe, denominated cartilaginei, which, as their name Imports, have a cartilaginous instead of a bony skeleton. This tribe was by Lin nzeus separated from the rest, on the mis taken idea, that the individuals which compose it were furnished both with lungs and gills, and should be ranked in the class of amphibious animals.
The genera-which pertain to the pre ceding orders are determined by the number of rays in the branchiostegous membrane, the condition of the teeth, the figure of the body, and of otber remark able parts. The characters of the species are taken chiefly from the number of rays in the fins, which differs in the dif ferent species. But as the precise enu meration of these rays is sometimes a matter of difficulty, and as they are like wise subject to variation, it is necessary to have recourse to other marks, and to adopt, as subsidiary characters, the form and situation of particular fins, the pro portion of the head to the body, the con dition of the lateral line, the number of the vertebra and ribs, &c.
Mr. Pennant describes fishes under the three great divisions of cetaceous, cartila ginous, and bony. The latter, which is by far the most numerous, he subdivides into four sections, entitled, agreeably to the Linnxan orders, apodal, thoracic, jugu lar, and abdominal.
The shape of the body of fishes is sub ject to considerable varieties. It is said to be compressed, when the diameter, from side to side, is less than from back to belly ; and depressed, on the contrary, when the diameter, from side to side, is greater than from back to belly. It is cylindrical, when it is circular in the greater part of its length ; ensiform, or sword-shaped, when the back and belly terminate in a sharp edge, or when the body gradually tapers from the head to the tail ; cultrated, or knife-shaped, when the back is somewhat flat, and the angle below acute; carinated, or keel-shaped, when the back is rounded, and the under part of the belly acute, through its length ; oblong, when the longitudinal diameter is much longer than the trans verse ; oval, when the longitudinal dia meter not only exceeds the transverse, but the base is circular, and the apex more acute ; orbicular, when the longitu dinal and transverse diameters are nearly equal; cuneiform, or wedge-shaped, when the body gradually flattens towards the tail : conical, when it is cylindrical, and grows gradually more slender towards the tail ; ventricose, when the belly is very prominent ; gibbous, when the back pre sents one or more protuberances ; annu lated, when the body is surrounded by rings, or elevated lines; articulated, when it is covered with connected and bony plates ; trigon, tetragon, pentagon, and hexagon, when the sides are plain, with three, four, or six longitudinal angles ; if the number of these angles exceed six, it is termed a polygon.