160 ANIMAL BIOLOGY 



its way, and then is forced out behind the edge of the op- 

 erculum. The more or less regular movements of the jaws 

 and operculum have to do with taking in water and forcing 

 it out past the organs of respiration. 



All of the fishes mentioned above have a bony skeleton 

 composed of a large number of separate bones. There is 

 a complex skull consisting of the cranium, or brain case, 

 the jaw bones and their supports, and various other parts. 

 Joined to the hind end of the skull is the vertebral column 

 consisting of numerous bi-concave vertebrae; each ver- 

 tebra has a dorsal arch, covering the spinal cord, and many 

 of the anterior vertebrae are connected below with ribs 

 which partially surround the body cavity. The rays of 

 the pectoral and pelvic fins are joined to bony frameworks 

 called respectively the pectoral and pelvic arches. 



One of the most peculiar of the internal organs is the so- 

 called swimming bladder, or "air bladder," which lies 

 in the upper part of the body cavity. This organ is filled 

 with gas secreted by the fish and serves as a sort of float. 

 In some species this air bladder communicates by a duct 

 with the esophagus. Among the lung fishes (Dipnoi) 

 the walls of this body are well supplied with blood vessels 

 and subserve the function of respiration. 



Most of the bony fishes produce a large number of eggs. 

 During the egg-laying period the females are usually 

 accompanied by the males and when the eggs are extruded 

 the males discharge their sperm, or "milt," over them 

 and thus effect their fertilization. In the breeding season 

 the males of many species, such as our common sunfishes, 

 are more brilliantly colored than at other times, and dis- 

 play themselves before the females in a manner that sug- 

 gests courtship similar to that practised by the males of 

 many birds. It is a common practice among fishes to 

 lay their eggs more or less indiscriminately and then, 



