.620 H T D R A. 
animal: “I liaveftudied.it,” fays he, “ ever fince June 
laft, and have found in it linking charableriftics of both 
plant and animal. It is a little aquatic being. At firft 
light every one imagines it to be a plant;'but, if it be 
a plant, it is lenfitive and ambulant; if it be an animal, 
it may be propagated by flips or cuttings, like many 
plants.” It was not till the month of March, in the fame 
year, that he could fatisfy himfelf as to their nature. 
When Reaumur faw, for the firft time, two polypes 
formed from one that he had divided into two parts, he 
could hardly believe his eyes ; and even after having re¬ 
peated the operation an hundred times, and again exa¬ 
mined it an hundred more, he fays that the fight was not 
become familiar to him. The firft account the Royal So¬ 
ciety received of the furpriling properties of the hydra, 
was in a letter from M. Buffon, dated the itSth of July, 
1741, to Martin Folkes, efq. their prefident, acquainting 
them with the difcovery of a fmall infeft called a po¬ 
lypus, which is found flicking about the common duck¬ 
weed, and which, being cut in two, puts forth from the 
upper part a tail, and from the lower end a head, fo as 
to become two animals inftead of one. If it be cut into 
three parts, the middlemoft puts out from one end a head, 
and from the other a tail, fo as to become three diftincl 
animals, all living like the firft, and performing the va¬ 
rious offices of their fpecies ; which obfervations are, adds 
Buffon, well averred. Mr. Folkes, in confirmation of 
the foregoing article, communicated to the Society a let¬ 
ter from the Hon. W. Bentinck, efq. at the Hague, dated 
September, defcribing the infecls difcovered by Trem¬ 
ble)', adding, that he himfelf had feen them. In Novem¬ 
ber, a letter was read from Dr. Gronovius, of Leyden, 
giving an account of a water-infeft not yet known to, or 
defcribed by, any author ; after defcribing it, he adds, 
“ but, what is more furprifing, if this animal is cut into 
five or fix pieces, in a few hours there will be as many 
animals, exaflly fimilar to their parent.” The accounts 
of this animal were fo extraordinary, that they were not 
credited until profeffors Albinus and Muffchenbroeck 
were provided with fome fpecimens, and found all that 
had been related to be exactly true. In March 1742, 
Mr. Folkes gave an account of them to the Royal Socie¬ 
ty, from obfervations made on feveral polypes which had 
been lent by M. Trembley from Holland to him. The 
infects now began to be known, and were foon found 
in England, and the experiments that had been made on 
them abroad were publifhed by Folkes, Mr, Adams, and 
Mr. Baker ; conviction now' became too ftrong for argu¬ 
ment, and metaphyfical objections gave way to facts. 
There are few animals more difficult to defcribe than 
the hydra, as it has icarcely any thing conftant in its form, 
varying continually in its figure : they are often fo bef’et 
with young, as to appear ramofe and divaricated, the 
young ones conftituting as it were a part of the parent’s 
body. Whoever has looked with care at the bottom of a 
\vet fhallow ditch, when the water is ftagnant, and the 
fun has bpen powerful, may remember to have feen many 
little tranfparent lumps, of a jelly-like appearance,, about 
the fize of a pea., and flatted on one fide ; the fame ap¬ 
pearances are alfo often to be feen on the under fide of 
the leaves of thofe weeds or plants that grow on the fur- 
face'of the water ; thefe are the hydra gathered up into 
a quiefcent ftate, and feemingly inanimate, becaufe either 
undifturbed or not excited by the calls of appetite to ac¬ 
tion. They are generally fixed by one end to fome folid 
fubftance, at .the other end there is a .large opening, 
round about which the arms are placed as fo many rays 
round a centre, which centre is the mouth. They are 
flender and pellucid, formed of a tender kind of fubftance, 
jn confidence fiomething like the horns of a fnail, and 
can conlraCl the body into a very fmall compafs, or ex¬ 
tend it to a confiderable length. They can do the fame 
with the arms ; with thefe they feize minute worms and 
various kinds of aquatic infects, bring them to the mouth, 
^uid fwallow them. After the food is digefted, and the 
nutritive parts which are employed in luftaining its 
life are feparated from the reft, they rejeCt the remainder 
by the mouth. 
The bulk of the hydra decreafes,in proportion as they 
extend themfeives, and vice verfa. They may be made 
to contract themfeives, either, by touching them, or a°i- 
tating the wa.tei; in which they are contained. They all 
contraCt themfeives fo much when taken outof the water, 
as to appear only like, a little lump of jelly. They can 
contract or extend their arms without extending or con¬ 
tracting the body, or the body without making any al¬ 
teration in the arms ; or they can contract or dilate only 
fome of the arms, independent of the reft; they can alfo 
bend their body and arms in ail poftible directions. They 
can alfo dilate the body at different places fome, fome- 
times 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 defcend at pleafure to the bottom of the water, 
and afcend again, either by the fides, or upon fome aqua¬ 
tic plants; they often hang from the furface of the water, 
refting as it were upon the tail; at other times they 
are fufpended by one arm from it. They walk alfo with 
eafe upon the furface of the water. If the extremity of 
the tail be examined with a magnifying glafs, a fmall 
part of it will be found to be dry, and above the furface 
of the water, and. as it were in a little concave fpace, of 
which the tail forms the bottom, fo that it feems to be 
fufpended on the furface of the.water on the fame prin¬ 
ciple that a fmall pin or needle is made to fwim. Hence, 
when a polype means to pafs from the fides of the glafs to 
the furface of 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 ; and thus it always does upon thefe oc- 
cafions. They attach themfeives fo firmly by the tail to 
aquatic plants, ftones, &c. as not to be eafily driven from 
the place where they have fixed themfeives; they often 
further Itrengthen thefe attachments by means of one 01: 
two of their arms, which they throw out-and fix to ad¬ 
jacent fubitances, as fo many anchors. 
The mouth of the polype, or hydra, is fituated at tho 
fore-part of the body, in the middle between the fliooting 
forth of the arms. The mouth aflumes different appear¬ 
ances, according to the different pufpofes of the infect; 
fometimes it is lengthened out, and forms a little conical 
nipple ; fometimes it appears truncated ; at other times the 
interval between the arms appears doled, fometimes hol¬ 
low : if it be obferved with a deep magnifier in either of the 
two laft cafes, a fmall aperture may be difcovered. The 
mouth of the polype opens into the ftomach, which is a 
kind of bag br gut that goes from head to tail ; this may 
be perceived by the naked eye, when the animal is ex- 
pofed to a ftrong light, or a candle placed on the oppofite 
fide to the eye ; for the colour of the polype does not de- 
ftroy the transparency thereof. The perforation, which 
is thus continued from one end to the other, is called the 
ftomach, becaufe it contains and digefts the aliments. The 
Ikin which inclofes the bag, and forms the ftomach, is 
the Ikin of the polype itfelf; fo that the animal may be 
laid to confift of but one Ikin, difpofed in the form of a 
tube or gut open at both ends. On opening the polype, 
no velfels are to be diftinguifhed ; and, whatever be the 
nature of its organization, it mult refide in the Ikin. 
The Ikin mull be fo far organized, as to perform all the 
operations neceffary for the nutrition and growth of the 
animal, without confidering thole that are neceffary for 
its various motions. Whatever are the means the author 
of Nature has emr’oyed for thefe purpofes, we are igno¬ 
rant of them. It' the Ikin be examined by a microfcope, 
it appears like lhagreen,, or as if it were covered with 
little grains; thefe are more or lefs feparated from each 
other, according to the degree in which the body is ex¬ 
tended or contraifted. If the lips of a polype be cut 
tranfverfely, and placed fo that the cut part of the Ikin 
may lie directly before the microfcope, the Ikin thrcugh- 
1 out 
