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THE POPULAR SCIENCE MONTHLY. 



may be, free air is so essential to many fishes that they die if pre- 

 vented from obtaining it, especially in hot weather. 



There is no organic reason why fishes could not breathe air if the 

 gills could be kept moist and free. Indeed, there are a few fishes 

 which even pass a great part of their lives out of water. " Such are 

 the two genera, both belonging to the Gobiidw, Periopthalmiis and 

 Boleopthalmus / these skip along close to the water-line on the sea- 

 shore, where they hunt for mollusks and insects. In their branchial 



Fio. 3. Eight Stages of the Development of toe Tadpole, from the recently hatched 



(1) to the Adult Form (8). 



cavity; like all fishes, they have true gills ; but these, though not dif- 

 fering widely from those of other fishes living constantly in the water, 

 are far from filling up the cavity, which is rather large ; and this 

 seems to contain not merely water but air as well."* It will be seen 

 that this respiration is analogous to that of the land-crabs. And no 

 better illustration is needed to show the identity in principle of the 

 two kinds of respiration, aquatic and aerial. 



Another class of air-breathing fishes, of which the Anabas scandens, 

 or climbing perch of India, is a famous example, have an upward ex- 

 tension of the branchial cavity containing complicated foldings of the 

 skull-bones covered with mucous membrane, which remains moist 

 either by secretion or by condensation of moisture from the air. The 

 Anabas lives in ponds which are liable in times of severe drought to 



* Karl Scraper's " Animal Life," p. 1S9. 



