144 VITAL FUNCTIONS. 



others, this function is performed by the skin. But in 

 those which are a step above these, we find some special 

 preparations for this function. In the Mollusca, the 

 organs of respiration are situated near the outer margin 

 of the shell, and consist of parallel filaments arranged 

 like the teeth of a fine comb. These are called branchia 

 or gills. 



Respiration of the Oyster. These organs are repre- 

 Fig. 88. 



sented by Fig. 88. Their mechanism when closely ex- 

 amined is exceedingly curious, and somewhat intricate. 

 There is a triangular canal d, which leads through the 

 whole length of the organ where it is attached to the 

 body. By means of this, the water is admitted to the 

 interior of the gills generally. Besides this provision 

 for the admission of water, by a sort of canal, there are 

 numerous small apertures, e, by which the fluid is sent 

 to every feather of the gills. The parts f /, are the 

 feathery extremities of this organ, which appear like a 

 treble fold of some fine fabric, suspended like a festoon. 



After the water, thus admitted into the branchia has 

 performed its office of aeration, it is again expelled by 

 a different opening. 



Another step in the scale of animal existence brings 

 us to the Fishes, the branchial apparatus of which is 

 much more complicated and important than that of the 

 mollusca. In these, the respiratory action is more essen- 



What are the respiratory organs called in the mollusca and 

 How is respiration performed in the oyster ? 



