of' certain Fishes of the Ganges. ^7 



run obliquely between these spines, and fill up the space be- 

 tween them, leaving merely a few irregular openings of com- 

 munication between the two sides above and below. 



In the Trichopolus Colisa it differs from that of the former, 

 in having a membranous septum in its abdominal portions; 

 thus dividing the organ into two compartments, from one ex- 

 tremity of it to the other. 



The air-bladder varies considerably as to figure, size, and 

 internal structure, among the species of the genera Pimelodus 

 and Silurus. 



In the P. Aor it usually consists of two portions of the figure 

 of a heart placed in a line, and united at their apices. 



It is extremely small in the P. Silondia in proportion to the 

 bulk of the fish. It lies close to the anterior vertebrae, is of an 

 oval figure, and is divided into two cavities, each of which, in a 

 fish weighing eight pounds, is not larger than a hazel nut. 



The P. Pangasius has it composed of four or more portions, 

 extending in a line from opposite the pectoral fins, to near the 

 end of the tail. The first is generally oval, the second pyra- 

 midal, and the two last, which run between the caudal portions 

 of the lateral muscles, approach to a cylindrical shape. The 

 numerous septa on its internal surface descend from above 

 downwards in the first portion ; in the second, they run in a 

 transverse direction ; and in the posterior ones, form a number 

 of irregular cells. 



In the Silurus Boalis it is of the figure of a heart, divided, 

 internally, by a longitudinal septum into two cavities, which 

 have a free communication with each other through a semilunar 

 opening at the anterior part of the septum.* 



It communicates in all of these species with the alimentary 

 canal, by a ductus pneumaticus, extending from its lower sur- 

 face to the oesophagus. 



The air-bladder, besides possessing the locomotive function 

 generally ascribed to it, appears, from the connection that ex- 

 ists between it and a set of small bones, analogous to the ossicuta 

 auditus, to exert some influence on the sense of hearing. 

 These bones correspond to the malleus, incus, and stapes of 

 Mammalia, and are present, according to Professor Weber, by 



• This cavity generally contains ;i number »>t' worms of the G. Fat 



