338 MR. F. DAY ON THE GEOGKAPHICAL 



Greographical Distribution of Indian Freshwater Fishes. — 

 I art II. The Siluridae. By Feancis Day, Esq., F.L.S. 



[Eead April 19, 1877.] 



In the first part of this paper' I gave an outline of the geogra- 

 phical distribution of the Acanthopterygian Freshwater fishes as 

 existing in Sind, India, Burma, Ceylon, and the Andaman Islands. 

 I now propose giving a similar sketch of the Sheat-fishes or Silu- 

 ridse", which form a large family amongst the Physostomi of Asia. 

 Mostly scaleless, their mouths are provided with sensitive feelers, 

 which, serving as organs of touch, assist them when seeking for 

 their prey in turbid waters. Vision in such localities would be 

 but of slight service ; and, as might be anticipated, their eyes are 

 comparatively small, whilst with advancing age these organs 

 atrophy, not increasing so rapidly as the remainder of the body. 



In addition to augmented facilities for feeling about in muddy 

 water, they have a considerable development of the auditory 

 organs, which doubtless must be of essential service. 



The air-vessel or swim-bladder in fishes is found possessing two 

 distinct functional ofilces. In the Acanthopterygians, destitute of 

 a pneumatic duct, its use (excluding the question of its connexion 

 with the internal ear) appears primarily to be a mechanical one, 

 viz. for the purpose of maintaining a required level in the water 

 and permitting the fish to rise or fall as desired. 



In the majority of Carps (Cyprinina), in addition to the fore- 

 going function, a pneumatic tube connects it with the pharynx, 

 or upper portion of the alimentary canal, and also a chain of ossi- 

 cles with the internal ear. In fact, it is employed both for hear- 

 ing and flotation. 



But in the Sheat-fishes (Siluridse) the power of employing the 

 air-vessel as a float appears to be subservient to that of hearing. 

 Living, as they do, a life of ground feeders, this organ is more use- 

 fully restricted to acoustic purposes. 



If we briefly consider where these fishes are found in Asia, it 

 assists us in understanding this. Almost absent from the clear 

 waters of the Eed Sea^, they commence being numerous at the 



^ Journal Linnean Society, Zoology, vol. xiii. p. 138. 



^ The Anacanthini, having no solely freshwater representatives, do not call for 

 remark. 



3 Arius thalassinus and Flotosus arab are the only two species of Siluroids 

 that haye been recorded from the Red Sea. 



