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gill-cavity. This mode of respiration appears to be a wise 

 provision of Nature, to enable fish in tropical countries, 

 during periodic dry seasons or in the rains, to migrate from 

 pond to pond in search of water wherein their natural food ex- 

 ists. Also as they ascend small water-courses to breed during 

 seasons of inundations, they are always liable to have the 

 supply of water suddenly arrested, and then they regain 

 rivers, &c., through muddy channels or moist pieces of grass. 

 Thirdly, there are some fish which also appear to swallow air, 

 and perhaps absorb oxygen through the skin, as the loaches, 

 Cobitidince, and the spined eels, Ithynchobdellidce, but no 

 special air-breathing apparatus has as yet been detected. 



^ESTIVATION OF PISH. 



XLIY. A curious phenomenon in Indian fresh-waters, 

 and one which has never been satis- 

 factorily explained, is the sudden 



appearance of healthy adult fish after a heavy fall of rain, 

 and in localities which for months previously had been dry. 

 When pieces of water inhabited by fish yearly dry up, what 

 becomes of them ? On 1 8th January 1869, when examining 

 this question, I was taken to a tank, of perhaps an acre 

 in extent, but which was then almost dry, having only about 

 four inches of water in its centre, whilst its circumference was 

 sufficiently dried to walk upon. The soil was a thick and 

 consistent bluish clay, from which, and not nearer than 30 

 paces to the water, five live fish were extracted from at 

 least two feet below the surface of the mud. They consisted 

 of two of the Ophiocephalus punctatus, and three of the Rhyn- 

 chobdella aculeata. All were very lively and not in the 

 slightest degree torpid ; they were covered over with a thick 

 adherent slime. Amongst the specimens of fish in the 

 Calcutta Museum is one of Amphipnous cuchia, which was 

 dug up some feet below the surface of the mud, when 

 sinking the foundation for a bridge. If when the water 

 failed fish invariably died, the tanks would be depopulated 

 the succeeding year, unless a fresh supply was obtained from 

 some other source, whilst the distance from other pieces of 

 water at which they re-appear excludes, in many instances, 

 the possibility of migration, which must always to a certain 

 extent be regulated by distance, time, and other local cir- 

 cumstances. Some species, especially " compound-breathers " 

 (para. XLIII), are able to live in liquid mud, which they 

 cannot employ for the purposes of aquatic respiration. The 



