J 62 



ICHTHYOLOGY. 



Introduc- to that of the expiratory functions of the hmgs in the 

 tion. higher classes, by not only separating excrementitious azote 



^'"V^"' and superabundant oxygen from the blood, but even dis- 

 charging those elements in such species as have this parti- 

 cular viscus provided with an air-duct- 



The more obvious use, however, of this organ seems to 

 be to maintain the fish in equilibrium, or to lighten or in- 

 crease its relative weight, so as to cause an ascension or a 

 sinking, in proportion as the bladder is compressed or ex- 

 panded. This is probably effected by the contraction or 

 dilatation of the ribs. At all events, it is certain, that when 

 the air-bladder bursts, the fish remains at the bottom, usu- 

 ally turning up its belly, and exhibiting other irregularities 

 in its locomotion. Another curious effect is observable in 

 regard to fishes which have been suddenly brought from a 

 great depth by means of a long fishing line, and which 

 having no time either to compress or partially empty the 

 organ in question, the air which it contains being no longer 

 pressed by the heavy weight of water, either expands so 

 as to burst the bladder, or by its dilatation forces the sto- 

 mach and oesophagus into the fish's mouth. M^hen the air- 

 bladder is pierced artificially, the fish almost immediately 

 turns upon its back, and sinks to the bottom. 



We have already alluded to the physiological opinion 

 which regards this organ as an auxiliary to the respiratory 

 system, and have likewise adverted to the argument against 

 that opinion, deduced from the fact of its being imperforate 

 in many species, and entirely wanting in others. We may 

 add, that Weber' has pointed out a remarkable connection 

 between the swimming bladder and the organs of hearing. 

 It would appear that the former in several instances sub- 

 serves the latter as a membrana tympani ; but its primary, 

 or at least most important purpose, seems to be to regulate 

 the ascending or descending movements. Though of the 

 highest importance in the structure of such species as pos- 

 sess it (and these are by far the greater number), yet the 

 swimming bladder is not indispensable in the general eco- 

 nomy of the class of fishes. In several genera (e. g. Pleu- 

 ronectes) it is entirely wanting, and the species in such 

 cases generally remain at the bottom, and, swimming ob- 

 liquely on one side, propel themselves forward by a nearly 

 vertical motion of the tail. In such cases both eyes are 

 on the same side, and the whole structure of the fish, espe- 

 cially the skeleton of the head, presents an unsymmetrical 

 aspect of a very extraordinary kind.^ In many cartilagi- 

 nous fishes, such as rays (commonly called skates), the ab- 

 sence of the swimming bladder seems compensated by the 

 enormous size of the pectoral fins, which, of all the exter- 

 nal organs, are probably the most efficient in raising the 

 body, as Uie caudal extremity is the power chiefly employ- 

 ed during an onward course. The lamprey, which has 

 neither swimming bladder nor pectoral fins, dwells in the 

 mud. Flat fishes being unprovided M'itli swimming bladders, 

 are supposed for that reason to raise themselves with dif- 

 ficulty to the surface ; and they do not appear to strike the 

 water laterally like other fishes, but swim rather after the 

 manner of the Cetacea, by a motion alternately up and 

 down. In all the other animals of tliis class the chief organ 

 of progressive motion is the tail, or prolongation of the 

 body, terminated by a caudal fin, the position of which, 

 unlike that of the great aquatic mammalia called whales, 

 is vertical. The reason of the difference is obviously this : 

 A true fish, possessing the power of extracting air from 

 water by means of its gills, does not (except at rare inter- 

 vals) require to mount to the surface for the performance 

 of the vital act of respiration ; but all cetaceous animals 



being furnished with lungs, which cannot perform their Introduc- 

 functions except through an immediate communication tion. 

 with the atmosphere, require their bodies to be terminal- ^'^^•"'•^ 

 ed by a horizontal expansion, the action of which is the 

 most efficient for an ascending course. 



It is, however, difficult to account for the fact that so 

 considerable an organ as the swimming bladder should 

 have been denied to so many species, not only of the 

 more indolent kinds, which dwell composedly at the bot- 

 tom of the waters, but to many others which yield to none 

 of their class in the ease and velocity of their move- 

 ments. Its presence or absence does not even accord 

 with the other conditions of organization ; for while it is 

 wanting in the common mackerel, it is found to occur in 

 a closely allied species, the Scomber pnetimatophorus of 

 Laroche. 



Another singular peculiarity connected with the organi- 

 zation of certain fishes may be also shortly noticed in this 

 place, we mean the power of conveying electrical shocks. 

 In Torpedos, the apparatus consists of membranous tubes 

 filled with mucous matter, divided by transverse cham- 

 bers closely set together, like the cells of honeycomb, 

 and disposed in two groups placed on each side of the 

 head. They receive enormous branches of nerves from 

 the fifth and eighth pair. In the Gymnotus this extraor- 

 dinary structure occupies the under surface of the body 

 throughout its entire extent, and to a considerable thick- 

 ness. It is composed oF parallel plates separated by thin 

 layers of mucilage. The effect of this natural galvanic 

 pile will be detailed in the course of the systematic por- 

 tion of this article, when we shall have occasion to men- 

 tion the electric fishes in their proper place. 



SECT. X. THE GENERAL POSITION AND RELATIONS OF 



FISHES, CONSIDERED AS A GREAT CLASS IN THE ANI- 

 MAL KINGDOM. 



It results not less from this general exposition of the 

 Structure of our present class, than from all observation 

 of special organization, that fishes form a class of animals 

 distinct from every other, and destined by the totality of 

 their conformation to live, move, and have their being in 

 the waters. The liquid element forms their proper place in 

 the creation ; there they had their origin, there they must 

 remain till the final consummation of all things, — and it is 

 either through slight and superficial approximations, or 

 by vain metaphysical speculation, that any modern wri- 

 ter could regard them as proceeding from an exalted or 

 more perfect development of the molluscous tribes. Equal- 

 ly unfounded is of course that other and corresponding 

 opinion, which, in the spirit of the same philosophy, looks 

 upon fishes as forming an elementary stage, or footal con- 

 dition, of the other vertebrated classes. It is true that 

 the Mollusca, in common with fishes, respire by means of 

 branchiae ; they equally possess a nervous and circulating 

 system, an intestinal canal and a liver ; " and nobody," 

 says Cuvier, with a justifiable pride, " knows these things 

 better than I, who was the first to make known with any 

 degree of completeness the anatomy and zoological rela- 

 tions of the molluscous tribes." As animal life, he con- 

 tinues,'' has received but a limited number of organs, it 

 necessarily happens that some of these organs are com- 

 mon to several classes. But where is in other respects 

 the resemblance ? The skeleton of these animals, and 

 their entire system of locomotion, are they comparable in 

 the least of their parts ? And even such organs as are 



' De Aure ct Auditn, &c. 



» In several insects of the genus Blatla we have observed a want of symmetry both in the size and markings of the elytra. >> e 

 do not mean an accidental variation of one side, but an evidently pre-ordained disparity of form and colour. 

 ' Hist. Nat. det Poitimi, t. i. p. 644. 



