FISHES 



121 



because of instinct, which is simply an inherited habit. 

 Rivers may be safer than the ocean for their young. They 

 are worn and exhausted by the journey, and never survive 

 to lay eggs the second time. 



Fig. 228. — A Shark {Acanthias vulgaris). 



The air bladder is developed from the food tube in the 

 embryo fish, and is homologous with lungs in the higher 

 vertebrates. Are their functions the same ? 



Fish that feed on flesh have a short intestine. Those 

 that eat plants have a long intestine. Which kind of food 

 is more quickly digested ? 



There are mucous glands in the skin of a fish which 

 supply a secretion to facilitate movement through the 

 water ; hence a freshly caught fish, before the secretion 

 has dried, feels very slippery. 



The air bladder, although homologous to lungs, is not a 

 breathing organ in common fishes. It is filled by the 

 formation of gases from the blood, and can be made 

 smaller by the contraction of muscles along the sides of 

 the body ; this causes the fish to sink. In the gar and 

 other ganoids, the air bladder contains blood vessels, is con- 

 nected with the gullet, and is used in breathing. Organs 

 serving the same purpose in different animals are said to be 

 analogous. To what in man are the gills of the fish analo- 

 gous ? Organs having a like position and origin are 

 said to be homologous. The air bladders of a fish are 

 homologous with the lungs of man ; but since they have 

 not the same use they are not analogous. 



