160 
NATURAL HISTORY. [NEW BUILDING. 
increases the size of the tube to fit it to the body, on which 
it is, as it were, moulded, and thus the tube assumes a 
tapering form. It is difficult to distinguish the tubes of 
some of these animals from the tubular shells of Mol- 
lusca, such as Vermetus , Magilus , &c., (some species of 
which Lamarck confounded with them,) unless the ani¬ 
mal can be examined ; but there is always one difference, 
that the animals of Molluscci are invariably attached to 
their shell by a strong muscle, and never quit it except 
at their death, while the Annelides only use it as a place 
of retreat, are not in any way attached to their shells, and 
often leave and form fresh ones, as occasion may require. 
These animals are generally provided with an ap¬ 
pendage on the side of the head, which bears on its end 
a calcareous operculum, used to close the mouth of the 
tube when the animal is contracted into it. The oper- 
cula vary greatly in shape in the different genera, and 
sometimes even in the individuals of the same species in 
the same group of shells. Turton described the oper¬ 
culum of one of the British species under the name of 
Patella tricornis. The operculum of the New Holland 
genus, Galeolaria, is very complicated, and furnished with 
a series of reflexed plates on the edge. 
In Cases 3 and 4 are exhibited for the present, until the 
Cases in the East Gallery are ready, a series of shells ex¬ 
hibiting the more prominent points in the economy of 
Moilusca, as— 
1. The great variations in size, the difference of sur¬ 
face and solidity, and the variations in external form pro¬ 
duced by the developement or non-developement of the dif¬ 
ferent processes in the individuals of the same species, all 
circumstances dependent on the quantity of food and the 
kind of locality they inhabit. 
2. The change that takes place in the form of the shell 
during the growth of the animal, as the expansion of the 
lip of the Strombidee and the Cerithice , the contraction and 
inflection of the lips of the Cypresides , and the deveiope- 
ment of the varices of the Muricidce. 
3. A series of section of shell to shew the form of the 
cavity, the form of the plaits, and the contraction of the 
size of the cavity of the shell to adapt it to the moving 
forward of the body of the animal, by the deposition of 
