168 SUPPLEMENT 
pretty nearly at this epoch, and its growth has continued. 
For the purpose of seeking its food, and placing itself in such 
circumstances as may be generally necessary to its develop- 
ment, it has been obliged to extend the different parts of its 
mantle, and especially the lobules, the strips, the digitations 
with which it is provided, and which are always more large 
proportionally, and even more numerous in youth than at the 
period of decrepitude, when they have a tendency to disap- 
pear. It is then that the edges of the aperture of the shell are 
extended and pass those of the retracted mantle, that the 
deposition of new strata augments incessantly, and so much 
the more that the animal, from some circumstance, is obliged 
to contract and retract itself still more. The shell has become 
a Shelter, a protecting organ, so much the better and so much 
the more complete as the animal has more approached to the 
summum of development of which it is susceptible. If the 
edges of the mantle were simple those of the shell are so 
likewise ; if, on the contrary, they were prolonged in any di- 
rection to facilitate some function, the edges of the shell have 
followed these prolongations, and similar prolongations result 
in the envelope. We must admit, however, that the pro- 
longations of the mantle had the organization necessary for 
excreting along with the mucous matter which the skin of the 
mollusca always throws out, a sufficient quantity of cretaceous 
matter. Without this it would be impossible to explain why, 
among the siphonobranches, there are species whose cuta- 
neous tube has produced a tube to the shell, as in the siphon- 
ostomata, and merely an emargination, as in the entomosto- 
mata. Itis thus we may explain not only the formation of 
the siphon, and of the spines which it has, but also that of 
the points or desections, more or less numerous, of the right 
edge of the aperture ofa shell, &c. As a general proposition, 
it is certain that the spines, tubercles, and prickles of a shell, 
however solid they may be, have at first been channelled; 
att - 
