36 CLASS XIV. 



water. This compression is effected "by motions of the ribs, whilst 

 in addition sometimes proper muscles are present in the Swimming- 

 bladder \ But in many fishes it is difficult to shew how they are in 

 a condition to expand the bladder and to rarefy the air. Whatever 

 opinion, however, be entertained respecting the use of the organ, it is 

 difficult to explain why, if it really exerts an important influence 

 on the life or the economy of fishes, it should be absent in so many 

 species, and even in some genera of fishes should be present in certain 

 species whilst in others of the same common form it is not found. 

 Certain it is that a bladder filled with air will render specifically 

 lighter the body of the fish, which, without it, is only a little above 

 the specific gravity of water ; and since it is placed near the back, 

 that part especially of the body which in other respects is the 

 heavier, so that the centre of gravity of the fish will sink, and its 

 turning belly upwards be prevented. Deep-bodied, compressed fishes 

 have also usually a well-developed swimming-bladder. According 

 to the experiments of Mueller, however, the fins and especially 

 the vertical fins (dorsal and anal fin) principally resist this upset- 

 ting. In some fishes, according to the discoveries of E. H. AVeber, 

 the swimming-bladder is in connexion with the auditory apparatus, 

 as an organ that intensifies and conducts sound. That this bladder 

 is a second respiratory organ and to be compared with the bladder- 

 like lung of certain reptiles, as some suppose, is sufficiently refuted 

 by the course of the blood-vessels ; for if this were true it would 

 receive venous and not arterial blood^. 



The secretion of urine is effected by two kidneys, lying 

 upon the spinal column, which are often united with each other 

 at their posterior extremity. In most osseous fishes they extend 

 from the head backwards as far as above the anus. They are usually 

 of a loose spongy tissue ; here the difference between internal 



^ In some of the Siluridce (AucJienipterus, Eaanemus, Sijnodontis, Doras and Mela- 

 pterurus) there is a bony lamina that descends obliqiiely on each side from the first 

 vertebra to the swimming-bladder, and compresses it like a spring. This lamina can 

 be raised by a muscle arising from the cranium, on which the bladder expands. See 

 J. MvelJjER Abha7idl. der Al:ad. d. Wissensch. zu Berlin, a. d. J. 1842. 



^ G-. Fischer modifies the opinion that the swimming-bladder is a respiratory 

 organ in this way : it is for the respiration of the air contained in the water, whilst the 

 gills serve for decomposing the water ; but in the respiration of fishes the water is not 

 decomposed, as has already been noticed above. 



