RESPIRATION IN FISHES. 



273 



consists simply in the opening and shutting of the 

 shell. By watching them attentively we may per- 

 ceive that the surrounding water is moved in an 

 eddy by these actions, and that the current is kept 

 up without interruption. All the Sepige have their 

 gills enclosed in two lateral cavities, which com- 

 municate with a funnel-shaped opening in the 

 middle of the neck, and alternately receiving and 

 expelling the water by the muscular action of its 

 sides. The forms assumed by the respiratory 

 organs in this class are almost infinitely diversified, 

 while the general design of their arrangement is 

 still the same. 



As we rise in the scale of animals, the respiratory 

 function assumes a higher importance. In Fishes 

 the gills form large organs, and the continuance of 

 their action is more essential to life than it appears 

 to be in any of the inferior classes : they are situated, 

 as is well known, on each side of the throat in the 





immediate vicinity of the heart. Their usual form 

 is shown at g g, Fig. 3G6, where they are repre- 



VOL. II. T 



