Fish Study. 741 



caudal fin never contains spines. Some fishes, like the stickle-back, have 

 the dorsal fin preceded by free spines, that is, spines not connected by a 



membrane. 



HOW A FISH BREATHES 



In order to understand how a fish breathes we must examine his 

 gills. In the throat just above the entrance of the gullet are several bony 

 arches, each bearing two rows of pinkish fringes. These are the gill 

 arches and the fringes are the gills. Into each fringe runs a tiny blood 

 vessel. As the water passes over the gills, the oxygen contained in it is 

 absorbed through the thin skin and the impurities of the blood are per- 

 mitted to pass out in the same manner. Since a fish cannot make use of 

 air unless it is dissolved in water, it is very important that the water in 

 an aquarium jar should often be replenished. 



The gill arches also bear a series of bony processes called gill rakers. 

 Their function is to prevent the escape of food. They vary in size ac- 

 cording to the food habits of the fish. A fish living upon minute forms 

 of life has long, fine gill rakers, which are placed close together. 



THE TEETH 



The shape, number and position of the teeth vary according to food 

 habits of the fish. Some fishes feed upon plants, others upon plants and 

 animals, and still others upon animal life alone. Most fishes, however, 

 feed upon other fishes. The commonest type of teeth are fine, sharp, 

 short ones arranged in pads. Such ones are found in the bullheads. 

 Some fish have blunt teeth suitable for crushing shells. Herbivorous 

 fishes have sharp teeth with serrated edges, while those living upon crabs 

 and snails have incisor-like teeth. One fish may have several types of 

 teeth. In some groups of fishes, teeth are entirely absent. Teeth are 

 borne on the jaws, roof of the mouth, on the tongue and also in the 

 throat. 



THE SENSES 



With the exception of smell the senses are not highly developed in 

 fishes. The sense of smell is located in the nostrils, the openings of which 

 may be seen on either side of the snout. The nostrils have no connection 

 whatever with the work of breathing. 



Fishes are usually near-sighted ; they have no eyelid, but the eyeball 

 is movable. This often gives the impression that the fish has winked. 



Extending along the side of the body from head to tail is a line of 

 modified scales containing small tubes connected with nerves ; it is called 

 the lateral line. It is believed to be connected in some way with the sense 

 of hearing. The fish has no ear-shaped hearing organs. 



The tongue of the fish is either bony or grisly and immovable. There 

 is very little sense of taste developed in it. 



