47° CONCH 
Of the Can- not only with regard to all land and fea fhells which 
Parts of ^ ave l°ft the firft turns of the fp.ire, and confequently 
Shells, &.c. thofe °f the tip ; but alfo in a great number of other 
— — y~«=«' marine teftaceous animals. It feems not only cer- 
tain, but even neceffary, that this reparation between 
tbe animal and the fhell (hould alfo take place in bi- 
valve fhells, if we take a diftinft and rational view of 
their growth. Whether this feparation is fuddenly ef- 
fected, or by gradual procefs, which is moil probable, 
it feems to be fufficiently obvious, by examining the 
internal furface of the valves. This is hill more 
hrongly confirmed by fawing univalve fhells, particu- 
larly thofe which are confiderably elongated and have a 
great number of turns in the-fpire, in a direction per- 
pendicular to their axis. In old fhells, feveral of the 
firft turns of the fpire will be found completely filled 
up with tehaceous matter, fo that the tip of the fhell 
has become quite folid,or atleaft it will appear to have 
been long unoccupied by any part of the body of the 
animal. But in tranfparent fhells, as in forne fpecies 
of helix, it is feen that this attachment does not exift ; 
and the H. planorbis can be preferved alive, although 
the tip of the fpire is broken off. 
Sect. III. Of the Colours of Shells. 
Inquiry cu- The infinite variety of the colours of fhells is one 
rious. of the moil finking parts of their hiilory ; and it be- 
comes a curious and interefling objeCl of inveftigation 
to inquire, whether thefe colours are uniform and con- 
"flant in the fpecies, and from what proceed this regu- 
larity and uniformity. The experiments and obferva- 
tions of Reaumur will affift us in this inveiligation. 
When a bole is made in a {hell, nearly at an equal 
diflance between its tip and opening, the new piece of 
fhell which is formed to flrut up the hole is ufualiy of 
a white colour, and often very different from that of 
the reft of the (hell. It would appear at firft that the 
new piece is of a different nature, and that it is not 
formed in the fame way as the reft of the fhell. To 
meet this difficulty, it will be neceffary to explain 
on what depends the regular variety of the colours 
of certain fhells : the fame experiments which lead to 
the diicovery of the caufe of the one, will ferve to un- 
fold the other. 
^ ^ This remarkable variety of colour is in no fhell more 
w from” par- remarkable than in the helix nemoralis. The ground 
ticu'ar cir- of this {hell is white, citron or yellow, or a com- 
cumftancek pound of different {hades of thefe colours. Different 
coloured rays are traced on this ground, turning fpiral- 
ly with the fhell •, in fome they are black, in others 
brown, and fometimes reddiih. The breadth of each 
of thefe rays gradually increafes as they approach to 
the opening of the fhell. It even fometimes happens, 
- that two of thefe bands are fo much extended in 
breadth, that they meet together and form one. Some 
individuals have five or fix of thefe bands, while others 
'have three or four, and even two, and fometimes only 
one. Others again have none at all, although of the 
fame fpecies •, and among the individuals which are 
marked with coloured bands, they are not always of 
the fame breadth, in the fame parts of the lbell • from 
which it appears, that no certain fpecific charafters 
can be derived from the colour, fince it is fubjeft to 
fo much variety. According to Reaumur, the vifcid 
3 
O L O G Y. _ Chap. V. 
and earthy matter of which the fhell is compofedls fecre-Of the Con- 
ted from the furface of the animal’s body 5 but in certain ftinient 
places of the furface, particles which produce a different ut 
colour are feparated ; and whether this depends cn a , &c '„ 
peculiar organization of thofe places, or on the form of 
the particles themfelves, it appears that thefe particles,, 
either of a different nature or of a different figure, by 
uniting, form bodies which refleft different rays of light j 
that is to fay, form parts of the lhell of different colours. 
This feems to be a neceffary confequence of the 
mode in which the growth of fhells is accomplifhed. Colounn g 
The whole external layer of the fhell is formed by cretetUlo® 
the neck of the animal, bccaufe it is that part which the neck, 
is neareft to the head, and confequently as the animal 
increafes in fize, 'this part ceafes to be covered with 
the old fhell. It, therefore, depends on this part of 
the animal to extend the fhell, and for this purpofe it 
is iufficient that the neck he furnifhed with glands fcr 
fecreting the different fluids, to form a fhell of differ- 
ent colours. If, for inftance, there are two or three 
glandular bodies' which fecrete brown or black parti- 
, cles, and that thefe glandular bodies are difpofed in a 
parallel dir eft ion to each other, while the glands on 
the reft of the furface only fecrete particles of matter 
which refleft the light of a citron colour, the fhell 
formed by thefe bodies will have a citron ground, with 
black or brown bands, nearly parallel, or which gradu- 
ally approach to each other, and become larger in the 
fame proportion as the external organs of the animal 
increafe in fize. 
If no fuch glandular ftrufture, or difference in the by 
matter fecreted, could be traced on the neck of the experiment. 
helix nemoralis, this explanation of the caufe of the 
variety of colours in fhells would appear extremely 
probable ; but this probability amounts to certainty, 
from the aftual obfervation of the exiftence of this pe- 
culiarity of ftrufture and efi'eft. When the helix ne- 
ntoralis is deprived of part of its fhell, the body ap- 
pears of a white colour, excepting towards fhe neck, 
where the white inclines to yellow, and where befides 
there is a number of black or brown bands, equal to 
that of tbe bands on the flrell, and arranged in the 
fame direftion. It has been obferved, too, that the 
individuals which have only one black ftripe on the 
fhell, have only one Angle black fpot on the neck j 
and thofe having four fpots on the neck, have four 
ftripes of the fame colour on the fhell. Thefe rays 
are placed immediately under thofe of the fhell ; they 
commence at the diflance of about a line from the ex- 
tremity of the neck, which is itfelf ufualiy fpotted 
with black all found. The exiftence, therefore, of 
thefe excretory organs can no longer be doubted. 
The difference of colour feems to prove the difference 
of ftrufture. But to eftablifh this beyond the poffibi- 
lity of doubt, it is only neceffary to have recourfe to 
experiment, by obferving what happens in the new 
piece of fhell which is renewed, in place of that portion 
which has been removed ; and if it appear that that 
part of the flrell which is formed oppofite to the black 
rays of the animal, is black, and if that which is 
formed between the ftripes be of a different colour 
from that of the ftripes themfelves on the reft of the 
body, no farther proof can be required. Now, it has 
been obferved, that that part of the new fhell formed 
on the neck oppofite to the black or brown ftripes on 
