ICHTHYOLOGY. 



Introduc- trunks proceeding from the fonr branchiae that the great 

 artery is formed which carries the blood to the viscera and 

 all the parts of the trunk, and which is by consequence 

 , the representative of the aorta of the Mammalia, — but of 

 an aorta wliich possesses neither auricle nor ventricle at 

 its base. Thus, according to Cuvier's views, the left cavi- 

 ties of the heart of quadrupeds do not exist in fishes, but 

 are replaced by a simple vascular apparatus, situate above 

 the branchiae, in like manner as the right cavities are placed 

 beneath them. 



SECTION VIII. — THE RESPIRATION OF FISHES. 



It is by an almost infinite subdivision of the vessels 

 over the surface of the branchias or gills, that the blood of 

 fishes becomes subjected to the influence of the circum- 

 ambient water, which is made to flow incessantly between 

 the branchiae by the movement of the jaws, and of the oper- 

 cular and hyoidean apparatus. This mode of respiration is 

 as necessary to fishes as the direct respiration of air is to 

 other animals. If the air is expelled l)y ebullition from the 

 water, fishes cannot live ; and many species are obliged to 

 rise frequently to the surface for the purpose of breathing 

 atmospheric air. It is easy to suffocate various kinds, by 

 keeping them beneath the surface, inclosed in a gauze net. 

 The absorption of oxygen, however, is comparatively small 

 among these aquatics, for it has been calculated that a man 

 consumes fifty thousand times more than is required by a 

 Tench. When fishes are deprived of water, they perish not 

 so much for want of oxygen, as because their branchiae be- 

 come dry, and unable to perform their functions duly. Hence 

 the species of which the branchial orifice is small, as the Eel, 

 or those which possess receptacles for moisture, like Anabas 

 and Ophicephalus, long survive exposure ; while such as 

 have their gills greatly cleft and open, as the Herring, ex- 

 ])ire almost instantly when withdrawn from their moist 

 abode. Some fishes, as Amphipiwus cuchia, and Sacco- 

 hrancluis singio, have a pulmoniform sac for supplying air 

 to the gills. 



SECTION IX. THE AIR-BLADDER OF FISHES. 



One of the most remarkable and characteristic organs of 

 fishes is the air or swim-bladder. In many genera it has 

 no opening or external communication, and in these the air 

 which it contains must be the result of secretion. It is 

 composed of an extremely fine internal tunic, of another of 

 a thicker texture and peculiar fibrous structure, remarkable 

 for producing the finest kind of isinglass, and is inclosed 

 .yithin the general coating with which tlie peritoneum in- 

 ^■ests the other viscera. It is sometimes simple, as in Perch, 

 sometimes furnished with more or less numerous append- 

 ages, as in some of the Haddock tribe, or branched, as in 

 certain ScicB7itB. Occasionally we find it divided, as it 

 were, into two or more parts, by a constriction, as in the 

 genus Cyprmus, many of the Siluridce, and others. It 

 is chiefly among the abdominal fishes that we find it com- 

 municating by a tube with the intestinal canal, and either 

 directly with the oesophagus, as in Ci/primis, or with the 

 l)ase of the stomach, as in the Herring. That of the 

 Sturgeon opens into the former portion by means of a 

 large orifice. Professor Owen states generally that the 

 contents of the air-bladder consist in most fresh-water fishes 

 of nitrogen, with a very small quantity of oxygen and a 

 trace of carbonic acid ; but that in the air-bladder of sea- 

 fishes living at great depths, oxygen predominates. Biot 

 found as much as 87 per cent, of oxygen in the air-bladder 



of deep-sea Mediterranean fishes. Dr Davy' in the air- 

 bladder of fresh-run Salmon found a trace of carbonic acid, 

 and 10 per cent, of oxygen, the remainder of the air being 

 nitrogen. Humboldt found 4 per cent, oxygen, and 96 per 

 cent, nitrogen, in the air-bladder of Gynmotus. That the 

 air-bladder is homologous with the lungs of the higher 

 animals, is the opinion of the best physiologist of the pre- 

 sent day. These in their embryo state have gills, the lungs 

 being developed when needed. In fishes the gills are per- 

 sistent, while the air-bladder is simple and embryonic, 

 having chiefly a mechanical function to perform. In the 

 Salmandroid d'anoidei, however, various Siluroids, and 

 Protopteri, the air-bladder shows some pulmoniform com- 

 plications. The Lepidosireii of the Gambia remains buried 

 in the mud during the dry season, and its respiration is per- 

 formed by pulmonary air-bladders, which have short, wide 

 tracheae, kept open by cartilaginous orifices ; and all the 

 oxygenation of tlie blood required during the torpid con- 

 dition of the fish is effected in tlie blood-vessels ramified 

 over the air-bladders. 



The more obvious use of this organ seems to be to main- 

 tain the fish in equilibrium, or to lighten or increase its re- 

 lative weight, so as to cause an ascension or a sinking, in 

 proportion as the bladder is compressed or expanded. This 

 is probably eftected 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, usually turning up 

 its belly, and exhibiting other irregularities in its locomo- 

 tion. 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 dilation forces the stomach and oeso- 

 phagus into the fish's mouth. When the air-bladder is 

 pierced artificially, the fish almost immediately turns upon 

 its back, and sinks to the bottom. 



Thovigh of the highest importance in the structure of 

 such species as possess it (and these are by far the greater 

 number), yet the air-bladder is not indispensable in the 

 general economy of the class of fishes.^ In some fishes it is 

 no bigger than a pea ; in several genera {e.g., Pleuronectes) 

 it is entirely wanting, and the species in such cases gene- 

 rally remain at the bottom, and, swimming obliquely 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, especially the skeleton 

 of the head, presents an unsymmetrical aspect of a very 

 extraordinary kind.^ In many cartilaginous fishes, such as 

 Rays (commonly called Skates), the absence of the swimming 

 bladder seems compensated by the enormous size of the 

 pectoral fins, which, of all the external organs, are probably 

 the most efficient in raising the body, as the caudal ex- 

 tremity is the power chiefly employed during an onward 

 course. The Lamprey, which has neither swimming bladder 

 nor pectoral fins, dwells in the mud. Flat fishes being un- 

 provided witli swimming bladders, are supposed for that 

 reason to raise themselves with difficulty 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 this class the chief organ of progressive 

 motion is the tail, or prolongation of the body, terminated 

 by a caudal fin, the position of which, milike that of the 

 great aquatic mammalia called whales, is vertical. The 

 reason of the difference is obviously this : a true fish, pos- 



1 Transacliuns of the Royal Society of Eflinburgh, xxi., p. 245, 1S55. - De Aure et Audita, &c. 



3 In several insects of the genus Jllaita we have observed a want of symmetry both in the size and markings of the elytra, 

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



VOL. XII. 2 P 



We do 



