32 MANUAL OF THE MOLLUSCA. 



fresh-water is immediately fatal to marine species, and salt-water 

 to those which properly inhabit fresh ; but there are some which ., 

 affect brackish water, and many which endure it to a limited ex- \ 

 tent. The depth at which shell-fish live, is influenced by the '^ 

 quantity of oxygen which they require ; the most active and i 

 energetic races live only in shallow water, or near the surface ; ] 

 those found in very deep water are the lowest in their instincts, j 

 and are specially organized for their situation. Some water- | 

 breathers require only moist sea-air, and a bi-diurnal visit' from 1 

 the tide, — like the periwinkle, limpet, and kellla ; whilst many 

 air-breathers live entirely in the water or in damp places by the 

 w^ater-side. In fact, the nature of the respiratory process is the 

 same, whether it be aquatic or aerial, and it is essential in each 

 case that the surface of the breathing-organ should be preserved 

 moist. The process is more complete in proportion to the ex- 

 tent and minute sub-division of the vessels, in which the cii'cu- 

 lating fluid is exposed to the revivifying influence. 



The land-snails {pnlmoniferd), have a lung, or au*- chamber, 

 formed by the folding of the mantle, over the interior of which i 

 the pulmonary vessels are distributed ; this chamber has a round \ 

 orifice, on the right side of the animal, which opens and closes | 

 at irregular intervals. The air in this cavity seems to renew ' 

 itself with sufficient rapidity (by the law of diffusion), without ' 

 any special mechanism. i 



In the aquatic shell-fish, respiration is performed by the | 

 mantle, or by a portion of it specialized, and forming a giU ' 

 [brancJiia). It is effected by the mantle alone in one family of i 

 tunicaries {pehnaiadcB), in all the bracJdopoda, and in one family ) 

 of gasteropods {actcEonldfB). \ 



In most of the tunicata, the breathing organ forms a distinct ■ 

 sac lining the muscular tunic, or mantle (fig. 8. b.); this sac has j 

 only one external aperture, and conducts to the mouth, which is ^ 

 situated at its base. It is a sieve-like structure, and its inner j 

 sm-face is clothed with vibratile cilia* which create a perpetual I 



* From cilium, an eyelash ; they are only visible under favourable circum- « 



