20 MOLLUSCA. 
classification. The external openings are those which are 
detected with the greatest facility, but the structure of the 
internal organs exhibits more varied and discriminating 
marks. 
ConDITION OF THE MoxiiuscA.—Molluscous animals, in 
reference to their condition, are divided, according to the 
situation in which they reside, into three groups, which 
may be termed terrestrial, fluviatile, and marine. Those 
that inhabit the land belong exclusively to the gasteropoda. 
Among these, some prefer open pastures, others the rub- 
bish of old walls, while not a few reside in woods or among 
dead leaves and putrid plants. All the animals of this 
group respire by means of a pulmonary cavity. 
The fluviatile mollusca, or such as reside in fresh waters, 
include not only many gasteropodous genera, but likewise 
a few belonging to the Conchifera. Amongst these, some 
breathe air by means of a pulmonary cavity, and come to 
the surface to respire. Such species frequent the more 
shallow ponds and lakes. Others, respiring by means of 
gills, are less dependent on the shallowness of the water, 
and consequently reside in different depths. 
The marine mollusca include genera of all the classes. 
Some burrow in the sand, or adhere to the rocks which are 
left dry by the receding tide. These are termed Littoral 
species. Others, however, which have been denominated 
pelagic, reside in the deep, and are seldom obtained but by 
dredging, or when thrown ashore during storms. 
The effect of temperature in regulating the distribution 
of molluscous animals, has not been investigated with any 
degree of care or success. Over the terrestrial and fluvia- 
tile species, it probably exercises a very powerful control, 
