AMPHIBIOUS VERTEBRATES 451 



mode of breathing is most frequently exemplified. This is partly 

 related to the fact that in the dry season the smaller streams of 

 such regions are liable to great reduction in size, and the habit 

 is also favoured by the great moistness of the air during the 

 wet season. The Snake-headed Fish (Ophiocephahis) of India, 

 for instance (fig. 563), possesses what is generally termed an 



^'g' S^2- — Indian Snake-headed Fish iOpkiocephalus) 



accessory gill-cavity covered by the upper part of the gill-cover, 

 but this cavity does not contain any gill-folds and is probably 

 to be looked upon as constituting a sort of lung. The habits 

 of these forms are described by Gunther (in The Study op Fishes) 

 in the following words: — "Like other tropical freshwater fishes, 

 they are able to survive drought, living in semi-fluid mud, or 

 lying in a torpid state below the hard-baked crusts of the bottom 

 of a tank from which every drop of water has disappeared. 

 Respiration is probably entirely suspended during the state of 

 torpidity, but whilst the mud is still soft enough to allow them 

 to come to the surface, they rise at intervals to take in a quantity 

 of air, by means of which their blood is oxygenized. This habit 

 has been observed in some species to continue also to the period 

 of the year in which the fish lives in normal water, and individuals 

 which are kept in a basin and prevented from coming to the 

 surface and renewing the air for respiratory purposes are suffo- 

 cated." The same kind of specialization is carried still further 

 in the Climbing Perch [Anadas scandens), where the cavity above 

 the gills has its lining raised into a number of folds, by which 

 the breathing surface is largely increased (fig. 564). This fish 

 is known to come out of the water and undertake comparatively 



