RKSPIRATION IN FISHES. 301 



in respiration, is usually expelled by a sepa- 

 rate opening. The part of the mouth through 

 which the water is admitted to the branchiaB is 

 sometimes prolonged ; 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 animal has withdrawn its body wholly within 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 Murex, JBuccinum^ and Stromhus ; 

 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 alter- 

 nately 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 consists simply in the 

 opening and shutting of the shell. By watch- 

 ing them attentively we may perceive that the 

 surrounding water is moved in an eddy by these 

 actions, and that the current is kept up without 

 interruption. All the Sepiae have their gills en- 

 closed in two lateral cavities, which communicate 

 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 



