116 
M O T 
•which is not the cafe with vegetables; for, however they 
may aft from a prefent impulle, their motions never ap¬ 
pear to be derived from any fource which may not be ac¬ 
counted llriftly mechanical. 
Though all animals are endowed with a power of vo¬ 
luntary motion, yet there is a very great variety in; the 
degrees of that power ; to determine which, no certain 
rules can be afligned ; neither can we, from the fituation 
and manner of life in animals, derive any probable reafon 
why the motion of one fhould differ fo very much from 
that of another. This difference does not arife from their 
lize, their ferocity, their timidity, nor any other property 
that we can imagine. The elephant, though the ftrongeit 
land-animal, is by no means the Howell in its motions ; 
the horfe is much fwifter than the bull, though there is 
not much difference in their fize; a greyhound is much 
fwifter than a cat, though the former be much larger, and 
though both live in the fame manner, viz. by hunting. 
Among infefts, the fame unaccountable diverfity is ob- 
fervable. The loufe and flea are both vermine, are both 
nearly of the fame lize, and both feed on the bodies of 
animals ; yet there is no comparifon between the fwift- 
nefs of their motions; while the bug, which is much 
larger than either, feems to have a kind of medium fwift- 
nefs between both. This very remarkable circumftarice 
feems not even to depend on the range which animals are 
obliged to take in order to procure food for themfelves : 
the motion of a fnail is flower than that of an earth-worm; 
while that of many caterpillars is much quicker than ei¬ 
ther; though we can fcarcely determine which of the three 
has the greateft or the lead extenfive range for its food. 
Of all animals the ffiell-fiffi move the llowefl, infomuch 
that l'ome have fuppofed them to be entirely deftitute of 
loco-motive powers; and mufcles particularly are denied 
to have any faculty of this kind. Every one knows that 
thefe animals can open and ffiut their Ihells at pleafure ; 
and it cannot efcape obfervation, that in every mufcle 
there is a flelhy protuberance of a much redder colour 
than the reft. This has been thought to be a tongue or 
probofcis, by which the animal takes in its food; but is 
in reality theinftrument of its motion from place to place. 
This protuberance is divided into two lobes, which per¬ 
form the office of feet. When the river-mufcle is inclined 
to remove from its ftation, it opens its ffiell, thrulls out 
this protuberance,and digs a furrow in the fand; and into 
this furrow, by the aftion of the fame protuberance, the 
fhell is made to fall in a vertical polition. It is recovered 
Out of this into the former horizontal one, by puffiing 
back the ftnd with the fame tentacula, lengthening the 
furrow, and thus the animal continues its journey by a 
continual turning topfy-turv'y. Marine mufcles perform 
their motions in the fame manner, artd by fimilar inftru- 
ments. In general they are firmly attached to rocks or 
{mail Hones by threads about two inches long, which are 
fpun from a glutinous fubftance in the protuberances al¬ 
ready mentioned. Other animals which inhabit bivalved 
Ihells, perform their motions by a kind of leg or foot; 
which, however, they can alter into almoft any figure they 
pleafe. By means of this leg they can not only fink into 
the mud, or rife out of it at pleafure, but can even leap 
from the place where they are; and this can be done by 
the limpet, which people are apt to imagine one of the 
moil fluggifti animals in nature. When this creature is 
about to make a fpring, it fets its ffiell on edge, as if to 
diminiffi friftion ; then, ftretching out the leg as far as 
poffible, it makes it embrace a portion of the ffiell, and 
by a Hidden movement, fimilar to that of a fpring let loofe, 
it ftrikes the earth with its leg,and aftually leaps to a con- 
liderable diftance. 
The Solen, razor or ffieath fiffi, is laid to be incapable 
of moving forward horizontally on the furface; but it 
digs a hole foinetimes two feet deep in the fand, in which 
it can afcend or defcend at pleafure. The leg, by which 
it performs all its movements, is lleffiy, cylindrical, and 
I O N. 
pretty long; and the animal can at pleafure make it af¬ 
lame the form of a ball. When lying on the furface of 
the fand, and about to link into it, the leg is extended 
from the inferior end of the ffiell, and makes the extremity 
of it take on the form of a ffiovef ffiarp on each fide, and 
terminating in a point. With this inftrument the animal 
makes a hole in the land; after which it advances the leg 
It 111 farther into it, makes it aflume the form of a hook, 
and with this, as a fulcrum, it obliges the fuel! to defcend 
into the hole. This operation is continued until the 
whole ffiell is covered; and, when the animal willies to 
regain the furface, it makes the extremity of the leg to 
aflume the form of a ball, and makes an effort to extend it. 
The ball, liovyever, prevents any farther defcent, and the 
re-aftion of the mufcular effort raifes up the whole fliell, 
which operation is continued until it reaches the furface ; 
and it is furprifing with what facility thefe motions are 
accompliffied by an animal feemingly fo little qualified to 
move at ail. See the article Solen. 
_ All other ihell-fifli, even thofe apparently themoftflug- 
giffi and deftitute.of any apparatus for motion, are found 
to be furniffied with fuch inftruments as enable them to 
perform all thofe movements for which they have any oc- 
cafion. Thus the fcallop, a well-known animal inhabit¬ 
ing a bivalved ffiell, can both fwim upon the furface of 
water and move upon land. When it happens to be de¬ 
ferred by the tide, it opens its fhell to the full extent, and, 
ffiutting it again with a fudden jerk, the re-aftion of the 
ground gives fuch an impulle to the whole, that it fome- 
times fprings five or fix inches from the ground ; and by a 
continued repetition of this aftion, it gradually tumbles 
forward until it regains the water. Its method of failing 
is ftill more curious. Having attained the furface of the 
water by means unknown to us, it opens the ffiell, and puts 
one half above water, the other with the body of the ani¬ 
mal in it remaining below. Great numbers of them are 
thus frequently feen failing in company with their ffiells 
flicking up above water when the weather is fine, and the 
wind afting upon them as fails ; but on the lead alarm 
they inftantly fhut their ffiells, and all fink to the bottom 
together. 
The oyfter has generally been fuppofed one of the moll 
Huggiffi animals in nature, and totally incapable of volun¬ 
tary motion ; but, from the refearches of the abbe Dicque- 
marre, this opinion feems to be erroneous. The oyfter, 
like many other bivalved ffiell-fiffi, has a power of fquirt- 
ing water out from its body; and this property may ea- 
fily be obferved by putting fome of them into a plate with 
as much fea-water as will cover them. The water is eject¬ 
ed with fo much force, as not only to repel the approach of 
ordinary enemies, but to move the whole animal back¬ 
wards or fidewife, in a direftion contrary to that in which 
the water was ejefted. It has been alfo fuppofed, that 
oyfters are deftitute of fenfation; but M. Dicquemarre has 
ffiown, that they not only poffels fenfation, but that they 
are capable of deriving knowledge from experience. 
When removed from fuch places as are entirely covered 
with the fea, when deftitute of experience, they open their 
ffiells and die in a few days; but, if they happen to ef¬ 
cape this danger, and the water covers them again, they 
will not open their ffiells again, but keep them fhut, as if 
warned by experience to avoid a danger fimilar to what 
they formerly underwent. 
The motions of the Echinus, or fea-urchin, are perhaps 
more curious and complicated than thofe of any other 
animal. It inhabits a beautiful multivaived ffiell, divided 
into triangular compartments, and covered with great 
numbers of prickles; from, which laft circumftance it re¬ 
ceives the name of the jea-ufckin, or fea-hedgo/iog. The 
triangles are feparated from one another by regular belts, 
and perforated by a great number of lioieSj from every 
one of which iflues a fleffiy horn fimilar to that of a fnail, 
and capable of moving in a fimilar manner. The prince 
pal ufe of thefe horns feems to be to fix the animal to rocks 
or 
