32 MxiNUAL OF THE MOLLTJSCA. 



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

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

 affect brackish w^ater, 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 

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

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

 and are specially organized for their situation. Some water- 

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

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

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

 water-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 circu- 

 lating fluid is exposed to the revivifying influence. 



The land-snails {pulmo7iifera), have a lung, or air-chamber, 

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

 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. 



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

 mantle, or by a portion of it specialized, and forming a gill 

 ihrancliia). It is effected by the mantle alone in one family of 

 tunicaries {peloiiaiada), in all the bracJiiopoda, and in one family 

 of gasteropods {actceonidcB), 



In most of the timicata, the breathing organ forms a distinct 

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

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

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

 surface is clothed with vibratile cilia* w^hich create a perpetual 



* rrom cilium, an eyelash ; they are ouly visible under favoui-able circum- 



