] 00 TELEOSTEI. PIITSOSTOMI. 



foul, or insufficiently charged with air, and take in air by the mouth ; 

 likewise we find that those which mainly take in atmospheric air 

 direct by the mouth may, to a certain extent, be able to use their 

 gills. If fishes having these two different modes of respiration are 

 placed in a globe of water, across which a diaphragm of net is in- 

 serted below the surface, so as to prevent their obtaining access to 

 the atmosphere, those of the first class which oxygenate their blood 

 at the gills are unaffected, whereas those which have accessory 

 breathing-organs and take in air direct, or amphibious fishes, die 

 from blood-poisoning. 



Amongst the Siluridce, Clarias has a dendritic apparatus attached 

 to the branchia} (see fig. 48, p. 114), and Saccobranchus has a long 

 air or respiratory sac passing backwards amongst the muscles of 

 the back from behind the gill-cavity proper. The blood from the 

 heart goes up the bulbus arteriosus, and divides into branches on 

 either side, one of which goes to each respiratory air-sac, the 

 anterior on the right, the posterior on the left side. The one 

 on the right goes along the upper wall of the sac, whilst that 

 on the left traverses the lower wall, giving off numerous lateral 

 branches. From this respiratory air-sac the purified blood is 

 returned by a vessel which conveys it direct to the aorta. 



It is thus evident that blood can be purified at these respiratory 

 air-sacs, and these fishes can be kept alive hours, and even days, 

 without being in water, thus enabling them to traverse considerable 

 distances where aquatic respiration would be impossible. They are 

 also able, when in water, to depurate some of the blood at the gills, 

 if occasion should render it necessary for them to do so. 



The air- or swim-bladder (not respiratory air-sac) exists in two 

 forms amongst the Siluridce. In marine species it is thick and not 

 enclosed in bone, likewise in most of the freshwater forms found 

 in the waters of the plains ; but as we approach the hills a change 

 occurs, and in most of the genera the air-bladder possesses an 

 osseous covering. 



Geographical Distribution. Tropical and subtropical seas and 

 rivers. Fishes of this family are very abundant in the fresh 

 waters of India, likewise in the estuaries and sea ; they are not so 

 common, however, in the clear waters around the Andamans. and 

 are nearly absent from the Red Sea. 



Uses. As a rule (excluding Pseudeutropius^ Callichrous, and Ailia), 

 the Siluroids are more eaten by the poorer than by the richer 

 classes, and for two reasons first, they are forbidden to Jews 

 and Mahomedans, and secondly, they are very foul feeders. Sacco- 

 brancTius and Clarias, however, are deemed nourishing and often 

 prescribed for patients recovering from illness. The air-bladders 

 of the marine forms are collected for export to China, as they 

 afford a coarse isinglass. 



