1050 FRESH-WATER BIOLOGY 



portions of a lake. Through the reduction in oxygen fishes may 

 be either killed in large numbers or compelled to emigrate. Fishes 

 being exclusive water animals are especially adapted to utilize the 

 oxygen in the water. The gills are the universally present res- 

 piratory organs but in special cases the fins and part of the ali- 

 mentary canal may be forced into service. In the gar-pike the 

 air-bladder serves as a lung, at least for the ehmination of CO2. 

 Various other fishes have cellular air-bladders cormected with the 

 alimentary canal that suggest respiration. Tower found that in 

 fishes dying of asphyxiation the ratio of CO2 to in the air- 

 bladder increases. 



During the breeding season when the gill chambers are full of 

 eggs much of the respiration of the bhndfishes is probably forced • 

 on the fins and general surface. In the surf-fish, to which the Sac- 

 ramento Hysterocarpus belongs, the young are born fully devel- 

 oped.^ In their earUest development in the ovary the general 

 surface of the larva must act as a respiratory organ, later the ali- 

 mentary canal functions as such. A continuous stream of ovarian 

 fluid passes in at the gill-opening and out at the anus at this time. 

 Finally the fins become hypertrophied into enormous sheets super- 

 abundantly supplied with blood vessels. In the Cuban blindfishes, 

 in which the young reach a length of an inch at the time of birth, 

 vascular lobes are developed in the ovary, which the young take 

 into their mouths and to which they cling, possibly both for food 

 and oxygen. It is very probable that those fishes that are capable 

 of living out of water for a time carry on respiration through their 

 moist skin. 



Temperature and Adjustment to it. — In nearly all fresh waters 

 of the temperate region there is a fluctuation in the water be- 

 tween 32° to 80° F. The extreme fluctuation is found only on 

 the surface of the water. In the bottom of lakes eighty feet deep, 

 the annual fluctuation ranges perhaps between 39° and 60° F. 

 Fishes can always escape the extreme fluctuations by seeking deeper 

 water. That they are adjusted to five through extreme cold is 

 shown by the fact that some species may be frozen in ice and re- 



1 The life history of this species has not been traced, but that of some of its 

 marine relations has. 



