STEUCTUEE AXT) PHrSIOLOGY OF THE MOLLFSCA. 



25 



X 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. 



Ilcspiratory system. The respiratory process consists in the 

 exj^osure 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 diu-ing 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 jiroperly 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 

 lowest in their instincts, and 

 are specially organised for 

 their situation. Some water- 

 breathers require only moist 

 sea air, and a bi- diurnal visit 

 from the tide — like the peri- 

 winkle, limpet, and J^ellia ; 

 whilst many air - breathers 

 live entirely in the water or 

 in damp j^laces by the water- 

 side. In fact, the nature of 

 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 {pidmonifera) have a lung or air-chamber, 

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

 the pulmonary vessels are distributed ; this chamber has a 



Fie:. 13 



Trlgoma pectinafa.'* 



* Trigonia pedinata, Lam. (original). Brought from Australia by the late Captnin 

 Owen Stanle}'. The giUs are seen in the centre through the transparent mantle. 

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



C 



