19 N THE GEELONG NATURALIST. 
ah 
-original shell. 
-circumstances. 
The shell itself is formed of carbonate of lime and a small 
«portion of animal matter, the mollusca obtaining this from its food. 
It has a small shell attached to it even before the egg is hatched, 
:and this is generally of a pale moss colour, and nearly transparent. 
As the. animal increases in bulk, it finds the shell is not large enough 
to cover it. Itadds to its dimensions by the aid of its mantle. The 
-animal, as before mentioned, has the power of causing a liquid 
matter to exude from this portion of its body, which is composed 
partly of membrane, and partly of carbonate of lime. ‘This, on 
exposure to the air or water becomes shell. The mollusca makes an 
addition to its shell by protruding the mantle beyond the edge, and 
-suffering it in that manner to remain exposed with the fluid which 
has been secreted, and this gradually and slowly assumes consistence 
fresh layers being added within, each projecting a little farther at 
the edge, and thus the new formation becomes similar to the 
By this means are formed what are called lines of 
growth. This operation is repeated at intervals as the animal 
increases in size. So by this beautiful method the shell and its 
inhabitant are always adapted to each other, and preserve their 
relative proportions. As the new portion of shell is always similar 
in shape, and in fact takes its form from that part of the mantle 
which is pushed out, it is evident that the form of every shell will 
„depend upon the manner in which the mollusca pro&udes its mantle, 
and it will be spiral, cup-shaped, and otherwise, according to 
This can be best explained by a few references. 
Thus the simplest form of shell is that of the limpet which has a very 
-small portion (represented at a) formed at the periodofits birth. As 
it increases in size, and finds this inadequate to protect its body, it 
` expands the border of- its mantle, and by degrees, enlarges its shell 
all round, until it presents the appearance of Az, in like manner it 
-continues to enlarge at different periods of its growth, until having 
.attained its full size, it ceases to require any further enlargement, 
' the shell is then mature. 
.growth, for in the limpet the mantle is extended equally all round 
This presents the simplest method of 
the original shell, and the size only is increased, and still preserves 
the same shape. But in the greater number of univalves the 
method of enlarging their shells is different, the mantle being more 
active on one side than on the other side of the aperture causes a 
spiral or turbinated shell, in which each successive portion of the 
spire is called a whorl. This is well shown in the scalaria, also its 
successive periods of growth. Dr. Grant, of the London University, 
sa s, “I am inclined to ascribe this spiral twist to the action of the 
heart and the respiratory organs, which are situated on the left side, 
and to the descent of the foot over the columella, in order to reach 
‘a solid surface to creep upon.” In afew genera the heart is situated 
