RESPIRATION OF EELS. 261 



Though the whole breathing apparatus of a fish is comprised 

 in a small compass, its surface if fully extended would occupy 

 a very considerable space that of the common skate, for in- 

 stance, being equal to the whole external surface of the human 

 body. This single fact may convince us of the numberless 

 ramifications and convolutions of the gills, and how wonderfully 

 Nature has contrived to multiply within very narrow limits the 

 points of contact between the minute bloodvessels and the 

 aerated water that flows along their sides. It may seem strange 

 that when fishes are taken out of their native element they 

 generally die almost immediately from want of air : such, how- 

 ever, is the case, for the gills being no longer floated out col- 

 lapse, and thus, by preventing the passage of blood through the 

 delicate branchial arteries, put a stop to the circulation as com- 

 pletely as strangulation could do. 



In some genera, however, a provision is made to permit of a 

 more lengthened existence out of the water, where the habits of 

 the fish render such an arrangement necessary. In the whole 

 tribe of eels, for example, the external fissure is removed very 

 far back and reduced to a very small vertical slit, converting the 

 cavity in which the branchiae are lodged into an elongated 

 chamber, in which a sufficient quantity of water can be retained 

 to allow respiration to continue for a considerable time in the 

 air. Thus the eels are able to travel during the night over the 

 moist meadows, in search of frogs or other suitable food, or to 

 change their situation. 



In several tropical fishes we find the gills communicating with 

 a cellular labyrinth containing water, which serves to keep them 

 moist. It is owing to such an apparatus, without which they 

 could not possibly exist during the long-continued droughts of 

 the arid season, that the climbing perches of India (Anabas), 

 the hassar of Gruiana, and the frogfishes of Ceylon are able to 

 wander overland in search of their natural element, when the 

 ponds or rivers in which they sojourned are dried up. An 

 admirable instinct teaches them to shape their course towards 

 the nearest pool, and the peculiar formation of their fins assists 

 them in their migrations. 



Thus in the frogfish the bones of the carpus form arms that 

 support the pectoral fins, which thus perform the office of feet ; 

 and the hassar, projecting itself forwards on its bony pectoral 



