20 SALMONID^ OF BRITAIN. 



even moderately clear and ho lias alarmed a trout, to observe tliat it first ceases 

 feediiig in order to watcli him; subsequontl}', if unsatisfied, it darts away, and 

 immediately all which may be within sight do the same, apparently alarmed at 

 the movements of the first fish. 



Before offering remarks upon respiration as effected among these fishes, 

 some observations appear to be desirable respecting their air- or swim- 

 bladders. This organ may be likened to a sac placed in the abdominal cavity 

 above the centre of gravity where it lies beneath the backbone or vertebral 

 column from which it is separated by the kidneys, while inferiorly the peritoneum 

 lies between it and the intestines. In the embryo it originates as an offshoot 

 from the dorsal surface of the upper portion of the alimentary canal, and 

 subsequently elongates into a blind tube, the terminal portion of which finally 

 enlarges into what constitutes the air-bladder. 



The air-bladder is generally considered to be homologous with the lungs* of 

 some of the higher orders of vertebrates, f being a sac with an outer fibrous and 

 glistening coat ; and an inner mucous and vascular one. A pervious pneumatic 

 duct, the remains of the embryonic structure, exists throughout life, connecting it 

 with the dorsal surface of the alimentary canal. Its uses are mainly mechanical, 

 as a hydrostatic organ or for the purpose of flotation as by a contraction or 

 distention of its walls it is able to conden.se or rarify the contained gases, thus 

 enabling its possessor to maintain a desired level in the water. The fish has 

 likewise the power of renewing, expelling, compressing or dilating its gaseous 

 contents, so that it can rise or fall as necessity or inclination dictates. It has 

 also acoustic iises, being partially employed for hearing, owing to its connection 

 with the internal eai'. 



During respii-ation the blood is oxygenated at the gills, by means of the 

 oxygen (of the atmosjjheric air which is normally present in water) and carbonic 

 acid gas passes off. For the purpose of breathing the fish takes or gulps in 

 water by its mouth, this passes backwards to the gills, and is then discharged 

 outwards by the gill-openings. Should anything prevent the water passing 

 as described, or should the delicate fringes of the gills become dry so that 

 they adhere together, or should they be choked by mud, the blood cannot be 

 oxygenated. While various causes, as heat, or the water being at a very high 

 elevation, may occasion an insufficiency of air being present in it, thus impeding 

 or even entirely stopping respii'ation, as would also be the case were the fish in 

 distilled water. Both an arterial and venous circulation are present, one for 

 nutrition, the other for excretion as by respiration or other means, while a jiortal 

 •system also exists. 



The heart is situated a short distance behind the lower jaw and between the 

 branchial and abdominal cavities, it consists of an auricle or atrium having thin 

 walls and into which a large venous sinus that is situated outside the pericardiac 

 cavity, and which receives the blood from the venous system, empties itself. 

 It also possesses a strong muscular ventricle and an arterial bulb, this latter 

 being a pear-shaped enlargement situated at the base of the arterial system and 

 termed the "bulbus arteriosus " which is destitute of conti-actile power. 



According to the experiments of Dr. J. Davy, when water in which trout were 

 ])laced was artificially raised to 75*^ or more they became very active and tried 

 to leap out ; on the contrary when a char was similarly treated with water of 80'^ 

 it went to the bottom wdth its head downwards as if seeking a cooler stratum. J 



* Among Dipnoids it is double, lung-like, and communicates throughout life by means of a 

 duct and glottis with the oesophagus or pharynx and on its ventral side. In the sturgeon this 

 organ does not appear to be employed for respiration, while it opiens on the dorsal surface of the 

 alimentary canal and is not guarded by a glottis. 



t Professor Albrecht, of Brussels, remarked {Pln/sioloriical Society of Berlin, Jan. IGth, 1885) 

 that " many naturalists were of opinion that the swimming-bladder was homologous with the 

 lungs, which likewise represented a tube in communication with the intestinal tract, an opinion 

 however decidedly opposed to the \'iews of the speaker, for in all fishes the swimming-bladder was 

 placed supra-intestinally on the dorsal side, while the lungs are invariably situated infra- 

 intestinally or on the ventral side of the intestinal canal." 



J In August, 1882, Dr. Davy {Plii/siulotiicat I'lesi'iinlu's) placed a common trout of about a 

 quarter of a pound weight into a good volume of water at 02°, which was pretty rapidly raised 



