276 TEXT-BOOK OF ZOOLOGY 



depth of a few fathoms, semi-darkness prevails, consequently the eyes 

 of fishes are generally remarkably large and their pupils very broad. 

 (Compare with nocturnal animals, e.g., owls and cats.) In animals that 

 live in the air the cornea is convex like a watch-glass, and, together with 

 the aqueous humour between the cornea and the iris, has a strong refrac- 

 tive action. In the eye of a fish, on the other hand, the refractive power 

 of the cornea is about the same as that of the water, and the surface of 

 the cornea is therefore flat. Eefraction in the eye of the fish is chiefly 

 due to the lens, which is almost spherical. When a fish is boiled the lens 

 becomes opaque, and then appears white. Eyelids which in other verte- 

 brates protect the eye from external injury, especially from dust, are 

 generally wanting in fishes. For this reason it is all the more important 

 for them that the cornea does not project from the surface of the body. 



The ear of fishes is a very simple structure. Neither external ear, 

 auditory passages, tympanum, or auditory ossicles are present, the sole 

 functions of these organs being to catch the waves of sound and transmit 

 them to the fluids of the auditory labyrinth (see Part I., p. 13). The fish, 

 however, can dispense with organs of this nature (compare with seal and 

 whale), since the waves of sound, which are propagated in water with 

 great ease, produce vibrations in the bones of the skull, which are trans- 

 mitted to the fluid of the labyrinth. 



The nose does not, as in air-breathing animals, take any share in the 

 function of respiration, hence it does not open into the interior of the 

 oral cavity, but consists merely of two pits placed at the front end of 

 the head, in which the water streams freely in and out. 



The whole of the integument functions as an organ of touch, the skin 

 of the lips, however, being more specially sensitive to tactile impressions, 

 and provided in many " bottom fishes " (sheat-fishes, tench) with fleshy 

 tactile filaments, or " barbels." 



The sense of taste, which has its seat in the mucous membrane of the 

 mouth, is but slightly developed. 



In many fishes (take a perch, for instance) a peculiar 

 dark line may be observed running along the middle of 

 each side of the body from head to tail. This is the so- 

 called lateral line. Each of the scales in this line is per- 

 forated by a tube opening on its surface and leading down 

 C'-" to a longitudinal canal which runs underneath the lateral 



SCALE OF THE ^ ne ' an( ^ ^ s provided with peculiar bodies which have all 



LATERAL LINE OF the appearance of sense organs. Indeed, naturalists are 



A ROACH, SHOW- . ,-, , .-, , ,, . ., 



ING THE CANAL. ot opinion that they represent the organs of a sixth sense, 



unknown (and absent) in ourselves. Possibly this 'sense 

 renders the fish cognizant of any excessive pressure of the water, since 



