Pisces. 163 



i 



The Fins. The ordinary fin has a set of fanlike rays 

 supported by a series of bones at the base of the fin. 

 Fins are designated as " soft-rayed " or " spiny-rayed," 

 according to the nature of the supporting rays. 



The dorsal, anal, and caudal fins are called median, as 

 they are in the middle plane of the body. The pectoral 

 and pelvic are spoken of as " paired fins," and are com- 

 parable to the two pairs of limbs of the higher vertebrates. 



Uses of the Different Fins. As already noted, the tail 

 fin is the chief propelling power. It also serves as a rud- 

 der in guiding the direction of movement. The paired fins 

 serve as balancing organs and also serve in elevating or 

 depressing the body. The dorsal and anal fins act like 

 the keel of a boat in steadying and guiding the movement. 



The Air Bladder. The/ air bladder is generally consid- 

 ered as comparable with the lung of the higher forms. It 

 certainly acts as an organ of respiration in the lungfishes 

 and some of the ganoids. 



But in most fishes the air bladder acts as an organ for 

 maintaining the fish's position in the water, and hence is 

 more appropriately spoken of as a " swim bladder." In 

 the lungfishes and some ganoids the air bladder has a wide 

 and direct connection with the gullet ; in many other fishes 

 the opening persists, but is less direct. On the other 

 hand, in many fishes the duct is entirely closed. The air 

 bladder may have thin walls or thick ; it may be in one 

 section or divided into several sections ; it may be attached 

 to the body wall or lie freely in the body cavity. 



Flatfishes. Fishes may be flattened in two ways: 

 (i) from side to side, that is, laterally, when they are said 

 to be " compressed," as in the ordinary fish, more mark- 

 edly shown in the fresh-water sunfishes; (2) a fish that 

 is flattened from above, or dorso-ventrally, is said to be 



