ANIMALS WITH A BACKBONE. 135 



A general idea of the two body-cavities, the nervous and visceral, 

 will be obtained by cutting the flsh through transversely. The ner- 

 vous cord is seen to lie above the vertebral column, the nervous canal 

 being formed by the interarching spinous process. Below the verte- 

 bral column is the large cavity containing the heart, stomach, etc., 

 while the rest of the section is occupied by muscles. 



The noises produced by certain fishes are due to the 

 action of the pneumatic duct or air-passage and swimming- 

 bladder (Fig. 143, S, a'), though different kinds of noises 

 are made accidentally or involuntarily by the-lips or the 

 bones of the mouth, as in the tench, carp, and a large num- 

 ber of other fishes. Over fifty species of fish are known to 

 produce sounds of some sort. The swimming-bladders of 



Fio. 142.— Swimmiiig-bladder (.'?. anterior, S'. posterior, division) of the bleak; 

 OS, oesophagus ; 2, air-passage of the air-bladder leading into the oesophagus. 



Trigia and Zeus have a diaphragm and muscles for open- 

 ing and closing it, by which a murmuring sound is made. 

 The loudest sounds are made by the drum-fish. In some 

 minnows, pouts, and eels the sound is made by forcing the 

 air from the swimming-bladder into the oesophagus. In 

 the sea-horse, the sounds are made by the vibrations of cer- 

 tain small voluntary muscles. 



It should also be noticed that the organs of hearing in 

 many musical fishes are said to be unusually well developed^ 

 hence these sounds are probably love-notes; and Abbot 

 notices the fact that these fishes are dull-colored during the 

 spawning season as well as at other timesj while Toiceless 



