[Illustration: FIG. 172.—Air-sacs of _Saccobranchus singio_. _a.b_, The air-bladder enclosed in its bony capsule; _a.c_, right air-sac; _a.s_, left air-sac; _c.a_, bulbus aortae; _l.a.v_, afferent vessel of the left air-sac; _r.a.v_, afferent vessel of the right air-sac; _r.e.v_, efferent vessel of the right sac. (After Hyrtl, altered by Hubrecht.)] With perhaps one or two exceptions, the accessory respiratory organs of Fishes seem to exist for the purpose of enabling their possessors to breathe in air. This is certainly the case with the labyrinthiform organs of _Anabas_ and its allies, and also in such Fishes as _Amphipnous_, _Saccobranchus_, and the Ophiocephalidae, and probably in others. Nearly all these Fishes are tropical in geographical distribution, more or less amphibious in their habits, and usually possess a remarkable capacity for sustaining life out of water, under conditions which are promptly fatal to ordinary Fishes. Thus, _Anabas scandens_ may be kept alive for days in earthen pots without water, and when free is able to travel short distances on land, especially in the early morning when the dew is on the ground, while _Amphipnous_ frequents {296}marshes, lurking in holes in the grass and about the sides of ponds. In fact, even when in the water, access to air, which is probably swallowed and passed over their accessory breathing organs, is indispensable to their existence. Experiments conclusively prove that if the Fish is artificially prevented from obtaining air in this way asphyxiation speedily ensues.[326] In addition to breathing air through the agency of special organs evolved for the purpose, there are many freshwater Fishes which, like those just mentioned, periodically rise to the surface and swallow air in order to saturate the water which bathes the gills with oxygen.[327] {297}CHAPTER XI THE AIR-BLADDER [Illustration: FIG. 173.—Transverse section of the body of a Teleost, to show the position of the air-bladder (diagrammatic). _a.b_, The air-bladder; _c_, coelom; _d.p_, ductus pneumaticus; _k_, the kidneys; _oes_, oesophagus; _p.p_ and _v.p_, parietal and visceral layers of the peritoneum; _r_, rib; _v.c_, vertebral column.]