HEAD. 37 



their great size indicating that the fish is either nocturnal, or 

 lives at a depth to which only a part of the sun's rays pene- 

 trate. On the other hand, small eyes occur in fishes inhabit- 

 ing muddy places, or great depths to which scarcely any light 

 descends, or in fishes in which the want of an organ of sight 

 is compensated by the development of other organs of sense. 

 In a few fishes, more particularly iu those inhabiting caves 

 or the greatest depths of the ocean, the eyes have become 

 quite rudimentary and hidden under the skin. 



In the ante-orbital portion of the head, or the Snout, Snout. 

 are situated the mouth and the nostrils. 



The Mouth is formed by the intermaxillary and maxillary Mouth. 

 bones, or by the intermaxUlary only in the upper jaw, and 

 by the mandibulary bone in the lower. These bones are 

 either bare or covered by integument, to which frequently 

 labial folds or lips are added. As regards form, the mouth 

 offers as many variations as the body itself, in accordance 

 with the nature of the food, and the mode of feeding. It 

 may be narrow, or extremely wide and cleft to nearly the hind 

 margin of the head; it may be semi-elliptical, semicircular, 

 or straight in a transverse hue ; it may be quite in front of 

 the snout {anterior), or at its upper surface (superior), or at 

 its lower {inferior), or extending along each side {lateral) ; 

 sometimes it is sub-circular, organised for sucking. The jaws 

 of some fishes are modified into a special weapon of attack 

 (Sword-fish, Saw-fish) ; in fact, throughout the whole class of 

 fishes the jaws are the only organ specialised for the purpose 

 of attacking ; weapons on other parts of the body are purely 

 defensive. 



Both jaws may be provided with sldnny appendages, har- 

 hels, which, if developed and movable, are sensitive organs of 

 touch. 



In the majority of fishes the Nostrils are a double opening Nostrils. 

 on each side of the upper surface of the snoiit; the openings 



