XVUl 



INTRODUCTION. 



of the head at the dorsal end of the first gill ; each pseudobranch consists of a small exposed 

 row of vascular filaments, or as "a vaso-ganglionic body, composed of parallel vascular 

 lobes, and covered by the membrane of the branchial chamber, as in Esox, Cypriniis, Gadiis. 

 In both cases the vein or efferent vessel of the pseudobranchia becomes the ophthalmic artery." — 

 (Owen.) This organ is small, but can be made out, where present, by the expenditure of a 

 little patience in dissection. 



The heart of a fish may almost be said to be in its mouth, so high up in the cavity 

 of the body is it situated. Its situation is in the throat behind the last branchial arch ; it 

 is separated from the abdominal cavity by a strong septum. A fish's heart differs from the 

 heart of another vertebrate animal by possessing only one ventricle and one auricle; the 

 latter receives the venous blood from all parts of the body, while the former propels it 

 through the branchial surfaces. From the ventricle there springs a large vessel called the 

 branchial artery, the base of which in most fishes is developed into a strong muscular cavity, 

 the bulbus arteriosus, which seems to serve as a kind of second ventricle. This branchial artery 

 carries the venous blood to the gills, where it is oxygenated by the water. From the gills 

 the blood is driven to a single dorsal artery {aorta), by whose branches it is conveyed to all 

 parts of the body, returning by the veins to the auricle from which it originally started. 



That curious piscine creature, the Lancelet, has no true heart; the circulation is effected 

 by certain pulsating sinuses. In all fishes, except in the Lancelet, the blood is red and 

 cold, that is, it has a temperature equal to that of the medium in which the fish dwells. 

 In the Lepidosiren the heart possesses two auricles and one ventricle. The mention of this 

 strange amphibian-like fish leads me to mention an organ that exists in many fishes, called 

 the "swim-bladder," or "air-bladder," with which every fisherman is familiar. This air-bladder, 

 which is very variable in form according to the species of fish, is a sac filled with gas; it 

 extends along the back of the abdomen, between the kidneys and the intestinal canal. 

 Sometimes, as in the Perch, it is .a simple elongated cylinder closed at both ends; some- 

 times, as in the Carp, Tench, Roach, and other Cypriuida;, this organ is divided crosswise 

 into two portions by a deep constriction, with a minute orifice leading from the one portion 

 to the other. Now this air-bladder is sometimes, and often, simply a closed sac ; but in 

 some fishes it opens into the oesophagus by a narrow channel or duct, called the diictii 

 pucuinaticus. How does the gas or air gain admittance into the bladder? In the case of 

 those fish whose air-bladders possess no ductus pncumaticus it seems that the gas must be 

 secreted by the inner membrane of the bladder from the blood; but in fishes which are 

 provided with a ductus pncumaticus, so as to lead to a communication with the gullet, the air 

 generated within may be in a great measure derived from the atmosphere. The contained 

 gas of the air-bladder of fishes consists of a mixture of nitrogen and oxygen with traces of 

 carbonic acid; in fresh-water fishes the largest' percentage of nitrogen occurs; in salt-water 

 fishes oxygen is said to occur in the largest proportion. Some, I believe, have maintained 

 that the gas of the swim-bladder of Carps consists of pure nitrogen. Humboldt, who experi- 

 mented on the Electric Eel, {Gymnotus e/cctricus), found the gas of its air-bladder to consist 

 of ninety-six per cent, of nitrogen and four of oxygen. Biot, on the other hand, experimenting 

 on some deep-sea Mediterranean fishes, discovered eighty-seven per cent, of oxygen, the rest 

 nitrogen with a trace of carbonic acid.* 



■•• The late Dr. Davy was rather doubtful as to the accuracy of these experiments. He remarks, "That the same 

 organ should secrete two gages so very different in their nature appears anomalous and deserving of further enquiry. 

 Indeed does not the entire subject need more minute enquiry? At present the facts relating to it are few, and seem 

 far from adequate to allow of very satisfactory conclu.sions being drawn as to the use of the bladder and its secretion 

 in the animal economy, except of a mechanical kind as affecting the specific gravity of the fish. Were the gas 

 uniformly of one kind, were it constantly azote, it might be easy to assign it a plausible end; the function of the 

 air-bladder might be inferred to be auxiliary to that of the kidneys. The secretion of oxygen is the anomalous fact, 

 so contrary is it to the ordinary changes in living animals in which the general tendency is to the consumption of 



i" 



