BOOM V.] NATURAL HISTORY. 87 
of the Serpula and allied genera are hard, like shells, 
and formed in the same manner by the deposition of a 
quantity of animal matter mixed with a chalky secretion, 
proceeding from the glands on the surface of the animal. 
As the animal enlarges in growth, and requires a larger 
case for its protection, it continually adds new layers of 
this matter near the end of the shell where its head is 
placed, and drawing its enlarged body forwards, gradually 
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 Mollusca 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¬ 
etta 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 trlcornis . 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 
Mollusca, as— 
1. The great variations in si£e, 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 
