186 THE RIVER-SIDE NATURALIST. 



acid not only by their gills, but by the whole surface of their 

 body as long as they are surrounded with water impreg- 

 nated with atmospheric air ; and the respiration of a great 

 many fishes will continue in the air and will absorb oxygen 

 if the gills are kept constantly moist. So that, as Muller 

 observes, aquatic respiration differs from respiration in the 

 air less essentially than might at first appear. A moist 

 surface is necessary even in pulmonary respiration. 



A few words as regards the fins organs of the utmost 

 importance in fish. Gunther divides the fins into vertical 

 or unpaired and horizontal or paired. He says : " Any of 

 them may be present or absent, and their position, number, 

 and form are most important guides in determining the 

 affinities of fishes." 



The vertical fins are the dorsal, caudal, and anal. 



Very important differences are to be found in the dorsal 

 fin, which is either spinous (acanthopterygian) as in the 

 perches, &c., or soft-rayed (malacopterygian) as in satmones, 

 &c. It may be single or divided. 



The caudal fin is seldom symmetrical, i.e., the upper 

 half is not equal with the lower. 



The anal fin may be single or plural, long or short, or 

 entirely absent ; and in some (the acanthopterygians) the 

 foremost rays are spined. 



The horizontal or paired fins consist of two pairs, 

 pectorals and ventrals. 



The pectoral fins are the homologues of the anterior 

 limbs of the higher Vertebrata, and are always inserted 

 immediately behind the gill-covers. 



The ventral fins are the homologues of the posterior 

 extremities, and inserted in the abdominal surface, either 

 behind the pectorals as in the Salmon, or below them as 

 in the Red Mullet, or in advance of them as in the Burbot. 



" The fins," says Gunther, " are organs of motion, but 

 it is chiefly the tail and the caudal fin by which the fish 

 impels itself forward. To execute energetic locomotion 

 the tail and caudal fin are strongly bent with rapidity 

 alternately towards the right and left ; whilst a gentle 

 motion forward is effected by a simply undulating action of 



