324 THE SEA SHOEE 



slimy skin that is apparently scaleless, but an examination with 

 the microscope will show that there are small scales embedded in it. 

 The dorsal and ventral fins extend to the tail, and the pelvics are 

 absent ; the gill-slits, which are very narrow, are at the base of 

 the pectorals. 



It might well be expected that eels would be possessed of some 

 form of accessory breathing apparatus, seeing that they can live so 

 long out of water, but this is not the case. They have, however, 

 a pouch-like gill-cavity which can be inflated and filled with 

 water by the fish, thus keeping the gills moist and functional. In 

 most other fishes the gill-chamber is not capable of holding water, 

 and thus the gills soon become dry and sticky, so that they adhere 

 together and fail to absorb the necessary oxygen when the fish is 

 out of water. 



Thus the Eel (Anguilla, vulgaris), in the remarkable migrations 

 for which it is noted, is capable of travelling over dry land for 

 considerable distances in search of suitable homes. 



If an eel be taken out of the water, these gill-pouches will 

 be seen to swell out almost immediately, and remain filled with 

 water as long as the fish is kept on land ; but when it is returned 

 to its natural element, it will at once discharge the water that kept 

 its gills moist, and which has become foul with the products of 

 respiration, and, with a few vigorous gulps, renew the supply. 



Eels spend their breeding season, which extends from July to 

 September, in salt or brackish waters ; and early in the following 

 summer, the young, which are now called elvers, and measure 

 from three to five inches in length, ascend the rivers, travelling 

 enorcnous distances and overcoming obstacles that we might well 

 expect to be insurmountable. Thus they perform two migrations 

 annually, though it is thought by some observers that the adult 

 never returns to the sea, but dies soon after it has deposited its 

 spawn. 



The family of Flat-fishes (Pleuronectidce) present many interest- 

 ing points of structure and habit in which they stand alone, the 

 variations in structure as compared with other fishes being due, of 

 course, to the habits which they have acquired. 



One of the first features that strike the observer on looking at a 

 flat-fish is the unsymmetrical form of the body. It is very much 

 compressed, and the fish having acquired the habit of lying on the 

 bed of the sea, sometimes on the left and sometimes on the right 

 side, the lower surface has become flattened more, and is of an 



