322 FISHES. 



ence, and we learn to associate fishes having very large 

 eyes with residence at considerable depths, at which light 

 is scarce and must be economised ; those, on the 

 other hand, with strikingly small eyes, often 

 embedded in the skin, with still greater depths, beyond 

 the range of light altogether. It is in their position, how- 

 ever, in which we find the greatest amount of variation in 

 the eyes of fishes. Normally they are situate on either 

 side of the snout ; in the adult flat-fish they are on the 

 same side, both on the upper, or coloured, surface of the 

 fish, the right or left eye travelling round to the opposite 

 side as mentioned under the division in question. In the 

 hammerhead, again, we find the eyes, of large size, at 

 either extremity of the "hammer." In the weevers the 

 eyes are, with the mouth, directed upwards, or, as the 

 inventors of trivial and scientific names have it, towards 

 the stars. In some sharks we find a loose nictitating 

 membrane ; and in mackerels and mullets there is present 

 a fatty eyelid. 



Without extending the province of this little book to the 

 consideration of internal anatomy, two characters of great 

 interest in the class before us must at any rate 

 ine * be mentioned, the lateral line and the air- 

 bladder. The lateral line, which is to be traced in the 

 majority of fishes as a curved black or white line along the 

 middle of the sides, but which is in many fishes absent 

 altogether, in others interrupted about half-way from the 

 head, is in reality a row of perforated scales through which 

 exudes the secretion from the mucous canal, so important 

 a factor in the free passage of fishes through the water. 

 Their bodies being lubricated with this matter to a still 

 greater degree than the much-discussed similar operation 

 in wildfowl, move through the water with a minimum of 

 friction, and almost, as it were, without (in our sense of 

 the word) getting wet. 



In the air-bladder } or, as it is often called, the swim- 

 ming-bladder, we have nothing more or less than an 



