CHAPTEE II. 



TOPOGEAPHICAL DESCRIPTION OF THE EXTERNAL PARTS OF 

 FISHES. 



In the body of a fish four parts are distinguished : the 

 head, trunh, tail, and the fins; the boundary between the 

 first and second being generally indicated by the gill-opening, 

 and that between the second and third by the vent. The form Form of 

 of the body and the relative proportions of those principal ^ ^ ^• 

 parts are subject to much variation, such as is not found in 

 any other class of Vertebrates. In fishes which are endowed 

 with the power of steady and more or less rapid locomotion, 

 a deviation from that form of body, which we observe in a 

 perch, carp, or mackerel, is never excessive. The body forms a 

 simple, equally-formed wedge, compressed or slightly rounded, 

 well fitted for cleaving the water. In fishes which are in the 

 habit of moving on the bottom, the whole body, or at least the 

 head, is vertically depressed and flattened; the head may be so 

 enormously enlarged that the trunk and tail appear merely as 

 an appendage. In one family of fishes, the Pleuronectidm or 

 Elat-fishes, the body is compressed into a thin disk; they 

 swim and move on one side only, which remains constantly 

 directed towards the bottom, a peculiarity by which the 

 symmetry of all parts of the body has been affected. A 

 lateral compression of the body, in conjunction with a length- 

 ening of the vertical and a shortening of the longitudinal 

 axis, we find in fishes moving comparatively slowly through 



