468 
Of the Con. 
lfituent 
Parts of 
Shells, &c. 
C O N C H O L O G Y. 
T, 79 
Reaumur’ 
experi- 
ments. 
So 
Procefs of 
the forma- 
tion of 
fliells, 
By fecre- 
t;on from 
the animal 
- It is in tlie firft way that the growth of vegetables and 
animals is accompliihed ; the fecond is the mode by 
which fhells receive new additions of matter, and en- 
large in fize. The firft is the mode of increafe peculiar 
to living, organized fubftances ; by the fecond, inor- 
ganized fubftances receive new additions of matter, 
and increafe in volume. Thefe indeed afford fufficient 
charaCteriftic marks for a natural divifion of bodies in- 
to two claifes, namely organized and inorganized fub- 
ftances. 
The experiments of Reaumur have decifively proved, 
that the growth of fhells is owing to the latter mode 
of increafe. Thefe experiments were made, not only 
on fea fhells, but alfo on land and river fhells ; on uni- 
valves and bivalves; and, in all, the refult was invariably 
the fame. In conducting thefe experiments, be inclo- 
fed the fhells, on the progrefs of which he made his 
obfervations, in boxes pierced with fmall holes, fo as 
to admit the water, but fo fmall as to prevent the egrefs 
of the animal. Thefe boxes were funk into the fea, 
or the river, and in this way he was enabled to watch 
the progrefs of the growth of the fhell. He firft obferved, 
that when the animal which exactly filled its fhell began 
to increafe its fize, the fhell in a fhort time not being 
fufhciently large to cover its whole body, part of it was 
naked or unprotected. This part of the animal muft 
always be towards the opening of the fhell, becaufe 
the fhell being previoufly completely filled, it cannot 
extend in any other direction. All animals which inha- 
bit fliells of a fpiral form, fuch as the fnail and volute, 
can only extend at the head, or the opening of the 
fhell ; whereas the animals in bivalve fliells, fuch as the 
muffel and the oyiter, may enlarge in their whole cir- 
cumference. In all the fpecies of teflaceous animals, 
it is this part which appears by the increafe of the ani- 
mal when it enlarges the fhell. This increafe takes 
place, according to Reaumur, by the following me- 
chanifm. 
It is a neceffary effeCt of the laws of motion, when 
liquids run in canals, that the fmall particles of thefe 
fluids, or the fmall foreign bodies mixed with them, 
which on account of their figure, or their lefs degree 
©f folidity in proportion to their furfac-e, move flower 
than the others, fly off from the centre of motion, and 
approach towards the fides of thefe canals. It even 
frequently happens, that thefe fmall particles attach 
themfelves to the internal furface of thefe canals or 
tubes, and form concretions of different degrees of 
thicknefs. It is befides certain, that the fluids which 
circulate in thefe tubes, prefs againft their fides on every 
point of their interior furface; fo that if they were 
pierced with a number of fmall holes of fufficient dia- 
meter to give paffage to the fmall particles of matter 
floating in thefe fluids, thefe particles would be depo- 
fited on the external furface, where a cruft would be 
formed, fimilar to that in the infide ; with this differ- 
ence, that it would become thicker and more folid, be- 
ing lefs expofed to the fri&ion of the fluid, than that 
which is depofited in the interior of the tube. 
To a fimilar mechanifm Reaumur afcribes the in- 
crcafe of fhells. The external furface of that part of 
the body of the animal which has extended beyond 
the limits of the old fhell, is furnifhed with a great 
number of canals, in which circulate the neceffary fluids 
for the nutrition of the animal, A great many fmall 
Chap. V. 
particles of a vifcid and earthy matter are mixed with Of the Con. 
thefe fluids. Now, as thefe particles are lefs fluid than ftituent 
thofe of which the liquids themfelves are compofed, shells S &f c 
they approach the fides of the veffels, which are them- ■ ’ i 
felves furnifhed on that fide of the external furface of 
the body of the animal, with a great number of pores, 
which allow them to efcape from tha veffels, fo that 
they are depofited on the external furface of thefe 
tubes, or rather in that of the body of the animal it- 
felf, which is uncovered by the fhell. 
Thefe particles of earthy and vifcid matter having 
reached the furface of the body of the animal, readily 
unite with each other, and with the extremity of the 
old fhell, efpecially when the excefs of moifture is dif- 
fipated ; and thus by their union they compofe a fmall 
folid body, which is the firft layer of the new addition. 
Other particles of fimilar matter continuing to efcape 
in the fame way from the excretory veffels of the ani- 
mal, form a fecond layer under the firft ; afterwards a 
third, and a fourth, or more, till the new part of the 
fhell has acquired fufficient confiftence and thicknefs. 
It is, however, obferved to continue thinner for a cer- 
tain time than the former opening, till the increafe of 
the animal requires another enlargement of its cover- 
ing- 
When a teftaceous animal is going to enlarge its 
fhell, as for inftance the common fnail, the body pro- 
jects from the opening. It is then feen to attach itfelf 
to a wall or fome other folid fubftance, and the portion 
of its body which is unprotected by the fhell, is foon 
covered with the fluids which are excreted from its 
furface. The pellicle which they produce when the 
fluid dries, is at firft thin andelaftic, but gradually af- 
fumes more confiftence, and becomes at laft fimilar to 
the old part of the fhell. If in this ftage of the pro- / 
cefs a bit of the fhell is broken and removed, without 
injuring the body of the fnail, the fkin of the animal is 
foon covered with a fluid, which gradually thickens, g2 
and becomes folid. Twenty-four hours after the opera- Timenecef- 
tion, a fine cruft may be obferved, which conftitutesf.wy to foira 
the firft and external layer, for repairing the breach 
which was made. At the end of fome days this layer 
has become thicker, and in io or 12 days, the new 
piece of fhell which is formed, has acquired the fame 
thicknefs as that which was removed. In making this 
experiment, certain precautions are neceffary, other- 
wife there is fome rifk of its failure. If, after the bro- 
ken piece of the fhell has been removed, and particu- 
larly if the fraCture is made near the edge of the open- 
ing, the animal is not fupplied with a fufficient quantity 
of nourifhment, its volume or bulk is foon diminifhed ; 
and now finding that what remains of the fhell is a 
complete covering to its diminifhed body, no excretion 
takes place for the production of a new portion. In 
removing fnails from a wall to which they had attach- 
ed themfelves, for the purpofe of obferving the progrefs 
of the formation of the fhell, fome days will efcape af- 
ter they are placed in the box, before the procefs com- 
mences, becaufe the teftaceous matter which had been 
already expended after fixing on the wall, muft be 
fully fupplied before any new portion can be again 
formed. 
This experiment {hows clearly, that fhells are only 
enlarged by receiving new additions of matter, after it 
has been excreted from the body of the animal, and 
not. 
