HYDRA. 
of this fungus, five with stems, and one 
without; these chiefly grow on decaying 
wood. 
HYDRA, in astronomy, a southern con- 
stellation imagined to represent a water- 
serpent. The number of stars in this con- 
stellation in Ptolemy’s catalogue is twenty- 
five, and in the Britannic catalogue, sixty- 
eight. 
HYDRA, polypes, in natural history, a 
genus of the Vermes Zoophyta class and 
order. Animal fixing itself by the base, 
linear, gelatinous, naked, contractile and 
furnished with setaceous tentacula or feel- 
ers ; inhabiting fresh waters, and producing 
its deciduous offspring or eggs from the 
sides. There are five species. H. gela- 
tinosa, minute, gelatinous, milk-white, cy- 
lindrical, with twelve tentacula shorter than 
the body : it inhabits Denmark, in clusters 
on the under side of Fuci. But on the 
viridis, the fusca, and the grisca the greater 
number of experiments have been made, 
by naturalists, to ascertain their true nature 
and very wonderful habits. They are gene- 
rally found in ditches. Whoever has care- 
fully examined these when the sun is very 
powerful, will find many little transparent 
lumps of the appearance of jelly, and size 
of a pea, and flatted upon one side. The 
same kind of substances are likewise to be 
met with on the under side of the leaves 
of plants that grow in such places. These 
are the polypes in a quiescent state, and 
apparently inanimate. They are generally 
fixed by one end to some solid substance, 
with a large opening, which is the mouth ; 
at the other, having several arms fixed 
round it, projecting as rays from the centre. 
They are slender, pellucid, and capable 
of contracting themselves into a very small 
compass, or of extending to a considerable 
length. The arms are capable of the same 
contraction and expansion as the body, and 
with these they lay hold of minute worms 
and insects, bringing them to the mouth, 
and swallowing them. The indigestible 
parts are again thrown out by the mouth. 
The green polype was that first discovered 
by M. Trembley: and the first appear- 
ances of spontaneous motion were perceiv- 
ed in its arms, which it can contract, ex- 
pand, and twist about in various directions. 
On the first appearance of danger they 
contract to such a degree, that they ap- 
pear little longer than a grain of sand, of 
a fine green colour, the arms disappearing 
entirely. Soon afterwards, he found the 
grisca, and afterwards the fusca. The 
bodies of the viridis and grisca diminish 
almost insensibly from the anterior to the 
posterior extremity; but the fusca is for 
the most part of an equal size, for two 
thirds of its length, from the anterior to the 
posterior extremities, from which it be- 
comes abruptly smaller, and then continues 
of a regular size to the end. These three 
kinds have at least six, and at most twelve 
or thirteen arms. They can contract 
themselves till their bodies do not exceed 
one fourth of an inch in length, and they 
can stop at any intermediate degree of ex- 
pansion or contraction. They are of vari- 
ous sizes, from an inch to an inch and a 
half long. Their arms are seldom longer 
than their bodies, though some have them 
an inch, and some even eight inches long. 
The thickness of their bodies decreases as 
they extend themselves, and vice versa; 
and they may be made to contract them- 
selves either by agitating the water in 
which they are contained, or by touching 
the animals themselves. When taken out 
of the water they all contract so much, that 
they appear only like a little lump of jelly. 
They can contract or expand one arm, or 
any number of arms, independently of the 
rest; and they can likewise bend their 
bodies or arms in all possible directions. 
They can also dilate or contract their 
bodies in various places, and sometimes ap- 
pear thick set with folds, which, when care- 
lessly viewed, appear like rings. Their 
progressive motion is performed by that 
power, which they have of contracting and 
dilating their bodies. When about to move, 
they bend down their heads and arms, lay 
hold by means of them on some other sub- 
stance to which they design to fasten them- 
selves; then they loosen their tail, and 
draw it towards the head; then either fix 
it in that place, or stretching forward their 
head as before, repeat the same operation. 
They ascend or descend at pleasure in this 
manner upon aquatic plants, or upon the 
sides of the vessel in which they are kept ; 
they sometimes hang by the tail from the 
surface of the water, or sometimes by one 
of the arms ; and they can walk with ease 
upon the surface of the water. On ex- 
amining the tail with a microscope, a small 
part of it will be found to be dry above 
the surface of the water; and, as it were 
in a little concave space, of which the tail 
forms the bottom ; so that it seems to be sus- 
pended on the surface of the water on the 
same principle that a small pin or needle 
is made to swim. When a polype, there- 
