STRrCTTJEE AND PHYSIOLOGY OT THE MOILUSCA. 25 



a large number of blood corpuscles. This lie regards as a proof 

 of the mixture of blood and sea water within the body of the 

 animal. 



Respiratory system. The respiratory process consists in the 

 exposure of the blood to the influence of air, or water contain- 

 ing air; during which oxygen is absorbed and carbonic acid 

 liberated. It is a process essential to animal life, and is never 

 entirely suspended, even during hybernation. Those air- 

 breathers that inhabit water are obliged to visit the surface 

 frequently; and stale water is so inimical to the water-breathers, 

 that they soon attempt to escape from the confinement of a glass 

 or basin, unless the water is frequently renewed. In general, 

 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 extent. The depth at which shell-fish live is probably 

 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 ^ \,,jp!*:^^' 



lowest in their instincts, and \ *\^^^^J|^^sc* 

 are specially organised for ^^^^/^^^m^^^b^ 



their situation. Some water- *",^^^1/^^^^^^^*^^l.. o 



breathers require only moist ^ ^^^^^^^^^^^^^®" — "^ * 

 sea air, and a bi- diurnal visit #^^^^^^^^^B|^^' ^ 

 from the tide — like the peri- '^^^^^^^^^H|^^' 



whilst 'many^ 'air-breather; ^^^^^^^^ 

 live entirely m the water or 42a2izi23S:i2i3i^^^ 



in damp places by the water- . 



Side. In fact, the nature of ^ i^ ^ 



the repiratory 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 extent and minute sub- 

 division of the vessels, in which the circulating fluid is exposed 

 to the revivifying influence. 



The land- snails {pulmonifera) 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 



* Trigonia pectinata, Lam. (original). Brought from Australia by the late Captain 

 Owen Stanley. The giUs are seen in the centre through the transparent mantle. 

 0, mouth ; 1 1, labial tentacles ; /, foot ; v, vent. 



C 



