AQUATIC RESPIRATION. 215 



Gasteropoda, or inhabitants of univalve shells, this opening 

 is usually wide. In the ^cephalu, or bivalve molhisca, the 

 gills are spread out, in the form of laminae, round the mar- 

 gin of the shell, as is exemplified in the oyster, where it is 

 commonly known by the name of beard. The aerated wa- 

 ter is admitted through a fissure in the mouth, and when it 

 has performed its office in respiration, is usually expelled by 

 a separate opening. The part of the mouth through which 

 the water is admitted to the branchiae is sometimes prolonged, 

 forming a tube, open at the extremity, and at all times al- 

 lowing 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 animal has withdrawn its body wholly within its shell.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, Buccinuui, and Sirombus; 

 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 Sepia: have 

 their gills enclosed in two lateral cavities, which communi- 

 cate with a funnel-shaped opening in the middle of the neck, 

 alternately receiving and expelling the water by the muscu- 

 lar action of its sides. The forms assumed by the respira- 

 tory 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 func- 



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 Ascidix. 



