CLASS PISCES 



349 



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

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

 and in some teleosts a narrow open duct still connects the two. Through 

 this duct water may enter and leave the air bladder. 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 



Fig. 238. — Diagrams to illustrate various types of caudal fins in fishes. A, diphycorcal 

 type (dipnoan, Protopterus). B, heterocercal 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 

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

 flying fish, Cypselurus) . 



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

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



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

 differences being correlated with the habits of the fish (Fig. 238). 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 common among extinct types. 

 The heterocercal 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 



