424 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



"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 labyrinthiforrn organs of Anabas and 

 its allies, and also in such Fishes as Amphipnous, Saccobranchus, and the 

 Ophiocephalidse, 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 scan- 

 dens 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 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 acces- 

 sory breathing organs, is indispensable to their existence. Experiments conclu- 

 sively prove that if the Fish is artificially prevented from obtaining air in this 

 way asphyxiation speedily ensues. 



"In addition to breathing air through the agency of special organs evolved 

 for the purpose, there are many fresh-water Fishes which, like those just men- 

 tioned, periodically rise to the surface and swallow air in order to saturate the 

 water which bathes the gills with oxygen." 



There is seen to be a great variety in the methods of attaining acces- 

 sory organs of respiration. Suppose that the conditions of life should 

 change, such that the descendants of these fishes should have to rely 

 wholly on a mode of respiration which was originally merely accessory. 

 Those forms in which the respiratory organ is developed in the gill 

 chamber, but only in connection with the fourth gill arch, would have to 

 undergo some degree of reorganization in the arterial connections of the 

 accessory organ, since the blood supply of the head is derived from the 

 first two gill arches. A reorganization which would connect the accessory 

 breathing organs with all of the afferent and efferent gill arteries would 

 be relatively simple, however, by virtue of the law of repetition of struc- 

 tures as well as the readiness with which cross-circuits are built up be- 

 tween closely associated blood vessels. The readiness of such variation is 

 exhibited in the genus Saccobranchus. In *S r . singio it has been noted 

 that the right and left respiratory sacs are supplied with blood from the 

 first and fourth afferent arteries respectively, whereas in S. fossilis both 

 air sacs are supplied by the fourth afferent arteries. The efficiency of the 

 originally supplemental apparatus as a substitute organ for oxygenation 

 would be most easily attained in such forms as Amphipnous cuchia, 

 where the air sacs are supplied by blood from the first afferent artery on 

 each side; but efficiency would appear to be almost as readily attainable 

 in other forms which use the pharyngeal chamber as the seat of the acces- 

 sory organs. 



