CLASS PISCES 



355 



coelom and which is a hydrostatic organ. It arises as an outgrowth 

 from the anterior portion of the aUmentary canal, and in the ganoids, 

 dipnoi, and in some teleosts an open duct still connects the two. The 

 organ tends to be divided into two chambers. The anterior chamber 

 contains in its wall a so-called red gland which takes oxygen from the 

 blood and passes it into the bladder; the posterior chamber is thin- 

 walled and permits reabsorption of gases by the blood. In this way 

 the amount of gases in the air bladder may be controlled. If more gases 

 are passed into the air bladder, the specific gravity of the fish is lessened 

 and it rises in the water. If, on the contrary, gases are removed, the 

 specific gravity is increased and the fish sinks. A fish, therefore, is 



Fig. 251. — Diagrams to illustrate various types of caudal fins in fishes. A, diphycercal 

 type (dipnoan, Protopterus). B, heteroeercal type (cartilaginous ganoid, sturgeon). 

 C, homocercal type (bony ganoid, Lepisosteus) . D. homocercal type (teleost, salmon). 

 E, homocercal type (higher teleosts). These figures show a progressive series. F, a 

 heteroeercal type representing a secondary modification for a particular purpose (teleost, 

 flying fish, Cypselurus). 



able to maintain its position in the water without muscular effort and 

 quietly to rise or sink in this manner. Some species, particularly bottom 

 forms, have no air bladder. 



380. Forms of Tails. — The caudal fins of fishes differ in shape, these 

 differences being correlated with the habits of the fish (Fig. 251). The 

 primitive type of tail is that which is evenly rounded dorsoventrally 

 and consequently termed diphycercal, or protocercal. This form is not 

 exhibited by many living fishes but is more common among extinct types. 

 The heteroeercal type in which the caudal fin is divided into two lobes, 

 one lobe being larger than the other, has been described for the shark. 

 This type of tail, when the dorsal lobe is the larger, gives greater strength 

 in swimming to strokes that would serve to direct the fish toward the 



