STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 25 
a jarge number of blood corpuscles. This he regards as a proof 
of the mixture of blood and sea water within the body of the 
aninal. 
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 watcr 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 1s 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 hive is probably 
influenced by the quantity of oxygen which they require; the 
most active and energetic races liye only in shallow water, or 
near the surface ; those found 
in yery deep water are the 
lowest in their instincts, and 
are specially organised for 
their situation. Some water- 
breathers require only moist PIX YS. 12 
sea air, and a bi-diurnal visit 22 2 oS 
from the tide—like the peri- 
winkle, limpet, and hkellia; 
whilst many air - breathers 
live entirely in the water or 
in damp places by the water- 
side. In fact, the nature of 
the repiratory process is the same, whether it be aquatic or 
aérial, 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 reyivifying 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 
| 
i) i 
a ae 
Me \ as 
I) 
ni 
Fig. 18. Zirigonia pectinata.* 
* Trigonia pectinata, Lam. (original). Brought from Australia by the late Captain 
Owen Stanley. The gills are seen in the centre through the transparent mantic. 
9, mouth; 7t, labial tentacles ; 7, foot; v, vent. 
Cc 
