404
which it again loofens, and retting on the tail, ftretches it out, as
in Fig. 1 q. It is eafy to fee from this account, that their manner
o f w a lk in g is very analogous to that o f various terreftrial and
aquatic animals. They walk very flow, often flopping in the
middle of a ftep, turning and winding their body and arms every
way. Their ftep is fometimes very Angular, as in the following
inftance: fuppofe the polype a b, Fig. 20, to be fixed by the tail
b, the body and arms being extended in the water, it firft bends
the fore-part towards the fubftance on which it is moving, and
fixes it thereto as at a, Fig. 21 ; it then loofens the lower end,
and raifes it up perpendicular, as in Fig. 22 ; now bending the
body to the other fide, it fixes the tail as in Fig. 23 ; then loofen-
jng the anterior end, it rifes up, as in Fig. 24.
They defcend at pleafure to the bottom of the water, and
attend again, either by the fides, or upon fome aquatic plants ;
they often hang from the furface of the water, retting as it were
upon the ta il; at other times they are fufpended by one arm from
it. They walk alfo with eafe upon the furface of the water. I f
the extremity of the tail b, Fig. 7, be examined with a magnifying
glafs, a fmall part o f it will be found to be dry, and above
the furface of the water, and as it were in a little concave fpace,
o f which the tail forms the bottom, fo that it feems to be fufpended
on the furface-of the water, on the fame principle that a fmall
pin, or needle, is made to fwim.
Hence when a polype means to pafs from the fides o f the glafs
to the furface o f the water, it has only to put that part out of the
water by which, it means to be fupported, and give it time to dry,
which it always does upon thefe occafions. They attach them-
' A * felves
felves fo firmly by the tail to aquatic plants, ftones, <ttc. as not to
be eafily driven from the place where they have fixed themfelves ;
they often further ftrengthen thefe attachments, by means of one
or two of their arms, which they throw out and fix to adjacent
fubftances as fo many anchors.
The mouth o f the polype, or hydra, is fituated at the fore-part
of the body, in the middle between the fhooting forth o f the
arms. The mouth afliimes different appearances, according to
the different purpofes o f the infeH; fometimes it is lengthened
but, and forms a little conical nipple, as at Fig. 13, Plate
XXIII. A ; fometimes it appears truncated, as at Fig. 8, Plate
X X I ; at other times the interval between the arms appears doled,
as at Fig. 2 and 12, Plate XXIII. A ; or hollow, as at Fig. 11,
Plate XXIII. A. If it is obferved with a deep magnifier, m
either o f the two laft cafes, a fmall aperture may be difcovered.
The mouth o f the polype opens into the ftomach, which is a
kind o f bag, or gut, that goes from head to tail; this may be
perceived by the naked eye, when they are expofed to a ftrong
light, or a candle placed on the oppofite fide to the e y e ; foi the
colour o f the polype does not deftroy the tranfparency thereof.
The ftomach will, however, be better feen, i f the eye be aflifted
by a deep magnifier; one o f them is reprefented as highly magnified,
at Fig. 8, Plate XXI. T o be fully fatisfied whether they
were perforated throughout, Mr. Trembley cut one trantverfely
into three parts; each piece immediately contrafled itfeif, and
' became very fhort, and being placed in a fhallow glafs full of watei,
and viewed through the microfcope, they were found to be vifibly
perforated. They are reprefented as they appear in the miciofcope,