FISHES 



241 



neath it. The free ventral edge extends far forward under the head almost 



meeting in a narrow throat region, the isthmus. 



t All the above features are adaptations for aquatic life, and, together with 



other internal organs, have made the typical fish unusually well suited to its 



environment. 



The general outline of most fish is about like the perch in having the flattened 

 sides and tapering posterior, which make for speed. All fish have the bulk of 

 their body composed of flexible muscle plates which permit powerful and free 

 use of the caudal fin in locomotion. 



There are two nasal cavities each with two nostrils, but they are 

 used for smell only, since they do not connect with the throat 

 and cannot be used in breathing. 



The eyes are large, somewhat movable, and have no lids, but 

 have a cornea, lens, retina, etc., somewhat similar to our own, 

 and are entirely different from the compound eyes of the insects. 



The ears are embedded in the skull and do not show externally; 

 they probably function as balancing organs and are used to detect 

 vibration, rather than sound, as fish have no sound-making apparatus 

 and probably cannot " hear " in the sense that we do. 



The Gills. At each side of the head is a crescent-shaped slit 

 which marks the rear 

 border of the gill cover or 

 operculum. These slits 

 almost meet on the ventral 

 side, leaving only a narrow 

 isthmus at the , throat 

 region, and thoroughly 

 exposing the gills to the 

 water. If we look inside 

 the mouth we can see that 

 the throat has five slits on 

 each side, leaving four 

 gill arches between them 



and if the operculum be lifted the outer sides of these gills can be 

 seen. 



Each gill consists of an arch of bone between the slits in the 

 throat wall, to which are attached two rows of thin- walled thread- 



FIG. 85. Fish structure of gill. 



