255 



considered as an oar of great strength, the mo- 

 tions of which impel the fish forwards, while the 

 fins, like so many secondary oars, facilitate and 

 direct its motions. 



The trunk of fishes includes the chest, belly 

 and tail. The chest which is very short, the 

 gills belonging to the head is separated from 

 the belly by a white and shining membrane call- 

 ed the diaphragm ; the belly is the portion of the 

 body placed between the chest and the tail, the 

 latter of which forms the termination of the 

 trunk. In some species the belly is wide and 

 thick, while in others it is narrow and slender ; 

 the back of some is much rounder than of others, 

 but in the greater number it forms an edge more 

 or less acute. The sides are placed between the 

 margins of the belly and back, and in most spe- 

 cies we find a mark, called the lateral line, run- 

 ning throughout their length. 



The fins are named according to the parts to 

 which they are appended : thus we have the dor- 

 sal, the pectoral, the ventral, the anal and the 

 caudal fins. The dorsal fin, situated on the back, 

 is either single, as in the pike ; double, as in the 

 perch ; or triple, as in the cod and haddock ; or 

 it may be altogether wanting. Some species, as 

 the salmon, possess an elongation of this fin, 

 to which the term adipose has been applied 

 The pectoral fins are placed on each side, near 

 the opercula or gill covers.. In some species they 

 are wanting, and others have two on each side. 

 In some species, as the flying fish, they are so 



2 L 



