Microscopical E s say s .
It has been already obferved, that it is to thefe grains that it owes
it’s fhagreen-like appearance; it is from them alfo that it derives
it’s colour; for when they are feparated from the polype, they
aie the fame colour with i t ; whereas the glarous matter is without
any diftinguifhing colour. The conflruction of the polype feems
then to be confined to thefe glandular grains, to the vifcous matter,
and the invifible fibres which a£t upon the glarous fubftance.
The ftrufture o f the arms of the polypes are very analogous to
that of their body. When they are examined by the microfcope,
either in a contrafted or dilated ftate, their furface is lhagreened;
i f the arm be much contrasted, it appears more fo than the body;
on the contrary, it appears lefs fo in proportion as they are more
extended, almqll quite fmooth when at their full extenfion ; fo
that in the hydra viridis the appearance o f the arms is continually
varying, and thefe variations are more fenfible towards the extremity
o f the arm than at it’s origin, as at Fig. to, Plate XXL
but more thinly fcattered, or farther afunder, in the parts further
on, as at Fig. 9, Plate XXL The extremity is often terminated
by a knob, the hairs which are exhibited in Fig. 9, cannot be
feen without a very deep magnifier, however they indicate a fur7
ther degree o f organization in this little animal.
All animals of this kind have, a remarkable attachment to turn
towards the light, and this might naturally induce the inquirer to
look for their eyes ; but how carefully foever this fearch has been
purfued, and however excellent the microfcope with which every
part has been examined, yet no appearance o f this organ was
found. . Notwi’thftanding this, they conflantly turn themfelves
towards the light; fo that i f that part o f the glafs in "which we
placed
M i c r o s c o p i c a l E s s a y s . 4 °9
placed them be turned from it, they will be found the next day
to have removed themfelves to the fide that is next the light, and
the dark fide will be quite depopulated.
O f t-he Food of the Hydræ, and their Method of
s e i z i n g and swallowing their Prey.
As the hydra fufca, Fig. 7, PI. XXI. has the longefl arms, it’s
manner o f feeding, and the different manoeuvres it makes ufe of to
feize and manage it’s prey, are more remarkable than thofeof the
two other fpecies ; it will be, therefore, this kind only which will be
principally fpoken of under the prefent head. T o view , them
properly, they (hould be placed in a glafs feven or eight inches
deep. If the polype is fixed near the top of the glafs, their arms
for the mod part hang down towards the bottom. This is a very
convenient fituation for giving them their food, and to obferve
how they manage it.
An hungry polype fpreads it’s arms as a filherman his nets;
it extends them every way, fo that they form a circle of confider-
able extent, every part o f which is entirely within the reach o f
one of them ; in this expanded pofture it lies in expectation of
it’s food; whatever comes within the’verge o f this circle is feized
by one or another of it’s arms. The arms are then contrasted till
the prey is brought to the mouth, when it is foon devoured.
While the arms are 'contrasting, and exerted vigoroufly, (to
counteraSl the efforts the animal which it has feized makes in
order to efcape) they may be obferved to fwell like the mufcles o f
the human body when they arè in a ftate of exertion.
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