300 THE VITAL FUNCTIONS. 



are only seven on each side. To these organs 

 are attached short and flat paddles, which are 

 moved by appropriate muscles, and are kept in 

 incessant motion, producing strong currents of 

 water, evidently for the purpose of obtaining the 

 full action of the element on every portion of the 

 surface of the branchiae. 



In the greater number of Mollusca, these im- 

 portant organs, although external with respect 

 to the viscera, are within the shell, and are 

 generally situated near its outer margin. They 

 are composed of parallel filaments, arranged like 

 the teeth of a fine comb ; and an opening exists 

 in the mouth for admitting the water which is 

 to act upon them.* In the Gasteropoda, or 

 inhabitants of univalve shells, this opening is 

 usually wide. In the Acephala, or bivalve mol- 

 lusca, the gills are spread out, in the form of 

 laminae, round the margin of the shell, as is 

 exemplified in the oyster, where it is commonly 

 known by the name of beard. The aerated 



* These filaments appear, in many instances, to have the 

 power of producing currents of water in their vicinity by the 

 action of minute cilia, similar to those belonging to the tentacula 

 of many polypi, where the same phenomenon is observable. 

 Thus if one of the branchial filaments of the fresh water muscle 

 be cut across, the detached portion will be seen to advance in 

 the fluid by a spontaneous motion, like the tentaculum of a 

 polype, under the same circumstances. Similar currents of 

 water, according to the recent observations of Mr. Lister, and 

 apparently determined by the same mechanism of vibratory cilia, 

 take place in the branchial sac of Ascidisc. 



