dilappearing entirely. He foon after found the hydra grifea, Fig.
»6, and law it eat, fwallow, and digeft worms much larger than
itfelf. This difcovery was foon followed by that "of the hydra
fulea, Fig. 7, Plate XXL
The moll general attitudes o f thefe hydras are thofe which are-
reprefented in Fig. 5 and 6, Plate XXI. They fix the pofterio?
extremity b again!! a plant, or other fubftance, as e f\ the body
a b, and the arms a c, being extended in the water. There , is a
fmall difference in the attitudes o f the three kinds-which we are.:
now defcribing.
The bodies o f the hydra viridis,. Fig. 5, and of. the hydra
grifea, Fig. 6, diminifh from the anterior to the pofterior. extremity
by an almoft infenfible gradation. The hydra fufca does
not diminifh in- the fame gradual manner, but from, the anterior
extremity a to-the part d, which is often two-thirds o f the length
o f their body, it is nearly ofan eq-ual fize ; from this part it becomes
abruptly fmaller, and goes on from thence o f a regular
lize to the end; The number of arms in thefe three kinds are at
leaf! fix, and at moil twelve or thirteen; though eighteen may
fometimes be found on the hydra grifea. They can contraQ. their
bodies till they are not above one-tenth of an inch in length ;
they can alfo flop at any intermediate degree; either o f contraction
or extenfion, from the greatefl to the leaf!. The fpecies
reprefented at Fig. 5 are generality about half an inch long
when ftretched out. Thofe exhibited at Fig. 6 and 7 are about
three-fourths o f an inch, or one inch, in length, though fome are
to be found at times about an inch and an half long. The arms,
of the hydra viridis,. Fig. 5, are feldom longer than their bodies,;
thofethofe
o f Fig. 6 are commonly one inch long, while thofe o f Fig.
7 are generally about eight inches; whence Mr. Trembley has
called it the long-armed polype.
The bulk of the hydra decreafes in proportion as they extend
themfelves, and vice verfa. They may be made to contraft them-
felves, either by touching them, or agitating the water in which
they are contained. They all contraa themfelves fo much when
taken out o f the water, as to appear only like a little lump of
jelly. They can contraa or extend their-arms without extending
or contracting the body, or the body without making any alteration
in the arms: or they can contraa or dilate only fome o f the
arms, independent of the reft; they can alfo bend their body and
arms in all pollible dircaions. Thofe reprefented at Fig. 7, let
their arms in general hang down, making different turns and returns,
often direaing fome of them back again to the top of the
water. They can alfo dilate the body at different places, fometimes
at one part, and then again at another; fometimes they are
thick fet with folds, which, if carelefsly viewed, might be taken
for rings.
They have a progreffive motion, which is performed by that
power by which they ftretch out, contrail, and turn themfelves
everyway. For iup pole the hydra, or polype, a b, Fig. 16,
Plate XXI. to be fixed by the tail b, having the body and arms a
extended in the water, in order to advance, it draws itfelf together,
by bending itfelf fo as to bring the head and arms down to
the fubftance on which it is to move; to do this, it fixes the
head, or the arms, as in Fig. '17 • when thefe are well fixed, it
loofens the tail, and draws it towards the head, as in Fig. 18,
3 C 2 which