I 
Chap. 'V. 
C O N- C H 
Parts of 
Shells, Sec. 
74 
Of the Con-of [hells, he charges Reaumur with fupporting the opi- 
ftituent n ] orij that teltaceous animals, when they proceed from 
eggs, are not furnifhed with the (hell, but that it is 
formed after being hatched. This opinion indeed has 
beeft aferibed to Reaumur by the hiltorian of the aca- 
demy, who, in the analylis of his excellent memoir on 
... the formation of (hells, has obferved, “ that hitherto 
miftaken' 011 cur * ous ^ ave been (brack with the prodigious varie- 
ty, the exaff regularity of drufture, the lingular beau- 
ty and fplendour of colour of (hells ; but naturaliils 
have been lefs attentive in (ludying and invedigating 
the mode of their formation. They feem to have 
thought that although (hells, as well as the covering of 
cruftaceous animals, are bones placed externally to the 
animals which they cover, it was neceffary to confide r 
them as part of their bodies, and to include this inex- 
plicable circumltance under that of the general forma- 
tion of animals, which is incomprehenfible to the hu- 
man mind. They have therefore fuppofed that the 
animal and its (hell proceeded from the fame egg, and 
were developed together ; and they have reded fatis- 
fied in admiring the economy of nature in providing 
fo elaborate a covering for fo low an order of animals. 
But this fuppofition, although probable, is not founded 
in truth. The animal only, not the (hell, is produced 
from the egg. The difeovery of this faff is owing to 
Reaumur.” 
It mud feem very extraordinary, that fuch an error 
fhould have, crept into the abdraft of the memoir of 
this celebrated philofopher, who in the courfe of it has 
clearly expreffed a contrary opinion. “ I have frequent- 
ly,” lays Reaumur, “ compared the (hells of fnails 
which were jud hatched, and even thofe which I 
had taken from the eggs before they were hatched, 
with other diells of full grown fnails of the fame fpe- 
cies, with which I had left only the fame number of 
whirls of the fpire with the fmall lhells, and then they 
appeared in all refpedts the fame.” He farther ob- 
ferves, “ that what has been faid with regard to the in- 
creafe of diells, renders it unneceffary to enter into the 
detail of their original formation ; for it is eafy to 
conceive, that when the body of a fmall embryo which 
is one day to fill a large (hell, has arrived at a certain 
(late, in which the different teguments in which it is 
included have lufficient confidence to fecrete from their 
AIL teftace- pores the peculiar duid which is dedined to the for- 
ou? animals mation of the (hell, this fluid may be depofited on the 
turn i (Tied f ur f acej ma y thicken, and at lad become firm and fo- 
lid. And thus commences the formation of the (hell, 
in the fame way as its increafe is continued. Snails do 
not proceed from the egg -without being previoufiy fur- 
nifhed with this (hell, which then has one turn and a 
little more of the fpire. 
When the eggs of tefiaceous animals are hatched, 
the young appears with its (hell already formed, and 
according to the obfervalion of Reaumur, it has then 
one complete turn of the fpire and a little more ; but 
at that period the (hell is extremely thin. It feems 
probable that the formation of the (hell is poderior to 
that of the principal organs of the animal, as the bones 
in the foetus of other animals are formed after the brain 
and heart. 
Reaumur has fufpe&ed that the (hell is the lad 
formed, and if proofs are wanting to edablifh this fadl, 
it is certain that at particular periods, if the eggs of 
ftituent 
Parts of 
Shells, &.C. 
75 
rvith the 
[hell be- 
fore bein 
hatched. 
75 
Shell laft 
formed. 
OLOGY. 467 
tedaceous animals are opened, the external parts of Of the Con 
the embryo are found already developed, without any 
appearance of the -fliell. But whatever may be the pe- 
riod of the formation of the fliell, it may be received 
as an eflabliflied faft, that the animal is furnifhed with 
it at the time, it leaves the egg. Leeuwenhoek fird 
obferved this faft with regard to cyders ; the fame ob- 
fervation was afterwards made by Lider, and extended 
to others, both landymd liver (hells. This obfervalion 
has been confirmed by other naturalids, and particular- 
ly by Rumphius, Swammerdam, Reaumur, and Adan- 
fon. From the invedigations of the latter it appears, 
that although there are many of the marine tedaceous 
animals which are viviparous, they referable thofe 
which are oviparous, in being furnifhed with the diell 
when they are feparated from the parent. *7 
Since then it appears, that the (hell of tedaceous Two opifii, 
animals is completely formed previous to the develope- ° ns 
ment of the animal, and that it may be cor.fidered as 7f- r ji ie i] 5> 
an effential part of its organization, let us now inquire 
into the mode by which its growth is effected. Ac- 
cording to the decifive experiments of Reaumur, the 
enlargement of diells is owing to juxta-pofition, or fuc- 
cedlve additions of earthy and animal matter, indepen- 
pent of any organized drudlure. Klein lias fupported 
a contrary opinion, and fuppofes that the growth of 
diells is effected by intus-fufeeption, or a kind of circu- 
lation. The opinion of Reaumur, however, has mod 
generally prevailed. Excepting Bonnet, few natural- 
ids have adopted that of Klein j and it will appear that 
this celebrated naturalid was led to entertain this opi- 
nion concerning the mode of the formation of diells, 
by the experiments of Heriffant on the generation of 
bone and (hell. From thefe experiments it was clearly 
demonilrated, that diells are compcfed of two fubftan- 
ces, the one a membranaceous or animal fubdanee, and 
the other an earthy matter ; but no fuch conclufion 
can be drawn from them in fupport of the opinion, that 
the (hell is a continuation of the body of the animal, 
or that it is fo clofely connedled as the bones in the 
bodies of other animals 5 or even that this connexion 
is formed by means of fibres of the ligament which 
attaches the animal to its (hell : for it has been fhovn, 
that thefe mufcular or ligamentous fibres, in all de- 
feriptions of tefiaceous animals, are fucceffively fepara- 
ted, in proportion to the increafe or enlargement of the 
died, This could not poflibly take place, if the evolu- 
tion and formation of the (hell, according to the opinion 
of Heriffant, depended on an internal circulation, ana- 
logous to what happens in the body of the animal. In 
this cafe the veffels which proceed from its body, hav- 
ing no longer a communication with thofe which are 
fuppofed to exid in the (hell, it would be deprived of 
nourifhment, and confequently could not increafe in 
fize. And it is found, that this feparation takes place 
in all (hells. It is gradually completed as the growth 
of the (hell advances. 
A body may increafe in volume in two different £ 0( ],^ s 3 or 
ways. Either the particles of which it is compofed K anized or 
pafs through that body by means of circulation, and inorgani- 
undergo certain changes by which they are prepared ^ed. 
to form part of the body or the particles of which a 
body is compofed, may unite with it by juxta-pofition, 
without any previous circulation or preparation within 
the body, to the increafe of which they are dedined. 
3 N 2 Ii 
