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Mollusca. 
Condition. 
MOLLUSCA. 
danger or irritated, and thus envelope themselves in a dark 
cloud, and elude the pursuit of their foes. A milky secre- 
tion is poured forth over the surface of the skin of some 
slugs when irritated. Other coloured secretions may like- 
wise be detected in the mollusca, to which we shall after- 
wards advert. The threadlike secretions, termed a byssus, 
with which some molluscous animals, especially among the 
Conchifera, fix themselves to other bodies, appear to be of 
an albuminous nature. A few species in this division have 
the power of secreting a /uminous fluid. Its nature, and the 
organs in which it is elaborated, have not been investigat- 
ed. It is probable that some animals, as those which have 
the faculty of raising or lowering themselves in the water, 
have likewise the power of secreting air into those organs 
which contribute to their buoyancy. 
Morbid secretions likewise occur amongst the animals of 
this division, chiefly, however, amongst the Conchifera. The 
most importaut of these are pearls, so much prized as orna- 
ments of dress. 
The organs of generation, some of which will be noticed 
afterwards in detail, furnish many important characters for 
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 Mottusca.—Molluscous animals, in 
reference to their condition, are divided, according to the si- 
tuation 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 wocds 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, 
greatly limiting their geographical range. In proof of this, 
it may be stated, that the south of France possesses several 
species not to be found in England, whilst in England, there 
are a few which have not been detected in Scotland. But, 
among the marine mollusca, the influence of climate is not 
felt in the same degree. Living in an element, the bulk 
and motions of which guard it equally from the extremes of 
heat or cold, these animals, like the sea-weeds, have a very 
extensive latitudinal and longitudinal range. Thus, some 
are common to Greenland and the Mediterranean, others to 
Britain and the West Indies. The mollusca of the tropical 
seas, however, differ widely as a whole from those cf the 
temperate regions. Some of the forms appear to be peculiar 
to warm regions, and, in general, the intensity of colour de- 
creases as we approach the poles. But as there have been 
few cultivators of this branch of science, the geographical 
distribution of the species has been but imperfectly explor- 
ed. Few parts of either England or Scotland have been 
surveyed by the eye of the helminthologist, so that many 
species, the range of which is considered as limited, may Mollusca. 
soon be found to be extensive. 
Ifthe observations are few and imperfect, which have been 
made on the influence of temperature, in regulating the 
physical distribution of mollusca, we are still in greater ig- 
norance with regard to the power of habit. In the fleetz 
rocks, the relics of marine and fluviatile mollusca are found 
mixed inthe same bed. ‘This circumstance gave rise to the 
inquiry, how far the mollusca of fresh water can be habitu- 
ated to sea-water, and vice versa. In the account of the 
ptoceedings of the National Institute of France, for the year 
1816, it is stated, that M.Benchant, professor at Mar- 
seilles, has directed his attention to this subject. He found, 
that all these animals die immediately, ifwe suddenly change 
their place of abode ; but that, if we gradually increase the 
proportion of salt in the water for the one set, and diminish 
it for the other set, we can, in general, accustom them to 
live in a kind of water which is not natural to them. He 
found, however, some species which resisted these attempts, 
and which could not bear any alteration in the quality of 
the water in which they reside. Before much confidence 
can be placed in the accuracy of these results, it would be 
desirable that the experiments were repeated and varied by 
other observers. There are, indeed, many sources of error 
to be guarded against. When we change animals from 
fresh to salt water, or from salt water to fresh, we must 
necessarily derange their motions, by compelling them to 
reside in a medium of a different degree of density from the 
one in which they have been accustomed to dwell, and to 
which the arrangement of the different parts of the body is 
adapted. By such a change of place, it would be difficult 
for those which breathe air to come to the surface, and de- 
scend again in their new situation. In those with gills, the 
application of a new kind of fluid to the surface of such de- 
licate organs, would considerably influence the function of 
respiration. The change of situation would likewise be 
accompanied by a corresponding change of food, and con- 
sequently, not merely the organs of locomotion and respira- 
tion, but likewise those of digestion, would suffer a derange- 
ment in their operations. We know that the power of suf- 
fering in the animals of this class is very great, and that 
they survive, though sadly mutilated. Some of the snails 
will live in a quiescent state for years, without food, and al- 
most without air. Unless, therefore, the animals subjected 
to these experiments of a change of situation, have been 
observed to thrive on the food which it spontaneously yields, 
to execute their accustomed motions, and above all, to pro- 
pagate their kind, we shall be disposed to conclude, that 
patient suffering has been mistaken for health and vivaci- 
ousness for the power of accommodation. : 
The influence of the seasons, in regulating the motions 
and habits of molluscous animals, has been but little attend- 
ed to. Those which live in the water, avoid the effects of 
low temperature, on the approach of winter, by retiring to 
the deeper parts of the lakes or rivers in which they reside. 
This migration, however, does not, in many cases, furnish 
the requisite security, so that they betake themselves to 
burrowing in the mud in which they repose until increasing 
warmth invites them to return to the open water. 
Among the naked terrestrial mollusca, it may be observ- 
ed, that they burrow in holes of the earth, under the roots 
of trees or among moss, and there screen themselves fron 
sudden changes of temperature, and appear to spend the 
winter in a state of torpidity. : 
The different kinds of shelly mollusca which inhabit the 
land, such as those belonging to the genera Helix, Buli- 
mus, and Pupa, not only retire to crevices of rocks and other 
places, for safety in the winter season, but they form an 
operculum or lid for the mouth of the shell, calculated to 
exelude the access of the air, and by the intervention of 
which they likewise adhere to the wall of their dwelling. A 
