ZOOLOGY. 367 



THE FUNCTIONS OF THE AIR-BLADDER. 



The physiological relations of the swiiumiug-bladder of fishes have 

 beeu considered by Professor Marangoni, in a special commimicitiou to 

 the French Academy of Sciences.* Some of the views enimciated were 

 quite novel ; whether they are entirely tenable remains to be seen. (1) 

 It was contended that one of the principal results of the development 

 of the bladder was exhibited in the migration of fishes ; those destitute 

 of that organ do not leave bottoms of little depth where there is tepid 

 water, while those possessing it, which live in deep, cold water, jieriodi- 

 cally seek the warmer surface water to deposit their ova. (2) If living 

 and dead fishes are i)laced in a vessel three-quarters full of water, and 

 the air be then compressed, it is seen that the dead fish fall downward 

 while the living ascend toward the surface, but if the air is rarefied the 

 contrary is the result. They suffer from the passive infiuence due to 

 hydrostatic pressure, and when brought from great depths the bladder 

 is often ruptured. (3) The development of the bladder entails on the 

 fish a double instability : (a) that of level, and {b) that of position, {a) 

 A fish with its bladder adapted to the conditions existing at a certain 

 depth may, through variation in pressure, be forced either upwards or 

 downwards, and is thus in unstable equilibrium as to level, {b) The 

 bladder being in the abdomen below the vertebral column, the centre 

 of gravity is above the centre of pressure, and the animal is consequently 

 constantly liable to inversion, and such inversion, in fact, results when 

 inaction or death supervenes. This two-fold instability compels fishes 

 to a constant gymnastic movement, and probably tends to conserve 

 their strength and agility. 



TEMPERATURE OF FISHES. 



It has been generally asserted that the temperature of fishes was little 

 above that of the surrounding element, except in the case of certain forms, 

 anch as representatives of the mackerel tribe — the tunny, bonito, &c. — 

 which have been stated to have a temperature of DOC" F. when the surround- 

 ing medium was 80,5°. This statement has, however, been challenged, 

 and Dr. Kidder availed himself of the opportunities oft'ered him while 

 attached as surgeon to the United States Fish Commission steamer in 

 3879, to institute investigations upon the subject.! All the observations 

 previously made, according to Dr. Kidder, had been confined to the intesti- 

 nal canal, the thermometer being placed into the rectum or oesophagus. 

 It is, however, by no means in either extremity of the intestinal canal 

 that we should look for the body temperature of a fish. In point of 

 fact the experiments show clearly enough that the rectum temperature 



"*Marangoni (C). Fonctions de la Vessie Natatoire des Poissous. Compics Rend. 

 Acad. Sc, Paris, t. xc, pp. 1293, 1294. 



t Kidder (J. H. ). Eeport of Experiments iiijou the Animal Heat of Fishes, &c. Proc 

 U. S. Nat. Mas., vol. ii, pp. 306-326, 1879. 



