272 THE VITAL rUNCTIONS. 



In tlie Gasteropoda, or inhabitants of univalve 

 shells, this opening is usually wide. In the Ace- 

 phala, or bivalve inollusca, the gills are spread out, 

 in the form of lamina?, round the margin of the 

 shell ; as exemplified in the Oyster, where it is 

 commonly known by the name of beard. The 

 aerated water is admitted through a fissure in the 

 mouth ; and when it has performed its office in 

 respiration, is usually expelled by a separate open- 

 ing. The part of the mouth through which the 

 M ater is admitted to the branchiae is sometimes pro- 

 longed ; forming a tube, open at the extremity, and 

 at all times allowing free ingress and egress to the 

 water, even when the animal has withdrawn its 

 body wholly within its shell. Sometimes one, and 

 sometimes two tubes of this kind are met with; and 

 they are often protected by a tubular portion of 

 shell, as is seen in the 3Iurex, Biiccimim, and Stroin- 

 bus; in other instances, the situation of the tube is 

 only marked by a deep notch in the edge of the 

 shell. In those mollusca which burrow in the sand, 

 this tube can be extended to a considerable length, 

 so as to reach the water, which is alternately sucked 

 in and ejected by the muscular action of the mouth. 

 In those Acephala which are unprovided with any 

 tube of this kind, the mechanism of respiration 



currents of water in their vicinity by tlie 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 por- 

 tion 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 Ascidise: but it is 

 doubtful whether they exist in the Cephalopoda. 



