hydra 
posed to the air, and thus making the instru- 
ment work with great rapidity. Cor Hydra. See 
eorl. Hydra tuba (pi. hydrtu I nbae), in Hydrozua, a stage 
in the development of certain Dixcophiira ; as a classifying 
name, a larval form of such acalephs, which was supposed 
to be a distinct animal. See xcyijhistonia. 
Hydrachna (hi-drak'na), n. [NL., < Or. Mop 
(vdp-), water, + axvq, foain, froth, chaff, the least 
bit (mite).] 1. A genus of aearids founded by 
Miiller in 1781, at present restricted to those 
fresh-water mites in which the third joint of the 
2932 
Hydraspididse 
branr, and consequently gives rise to very wa- hydrant (lu'drant), n. [< Gr. Map (Mp-), water, 
tery stools. (It) A remedy believed to be capa- + E. -an ft.'] An apparatus for drawing water 
ble of drawing off serum effused into any part 
of the body, as a cathartic of the above class 
or a diuretic. 
hydragogyt, [= Sp. hidragogia, < Gr. Mpa- 
loyia, a conducting of water, < Upaywfif, con- 
ducting water: see hydragogue.] The art of 
constructing aqueducts, or of conducting water 
through channels. 
Hydragogie demonstrated the possible leading of water 
by nature's law, and by artiflciall help, from any head (be- 
ing a spring standing or running water) to any other place 
assigned. Dee, fret, to Euclid (1570). 
hydra-headed (hi'dra-hed'ed), a. Having nu- 
merous heads, like the Lernoean Hydra ; hence, 
difficult of extirpation ; self -renewing; spring- 
ing up again after suppression, as abuses, vices, 
and the like. 
[< Hydra, 4, 
Never Hydra-headed wilfulness 
So soon did lose his Beat, and all at once, 
As in this king. Shale., Hen. V., 
Hydrachna belostoma. 
a, adult (line shows natural size) ; l>, mature larva, with pupa form, 
ing within (highly magnified). 
palpi is the longest, the beak is as long as the 
palpi, and the mandibles have sharp blades. 
These mites are parasitic upon aquatic insects, attaching 
themselves to species of Nepa, llanatra, Dytiscus, etc., 
during what may be called the pupa-stage. H. belostonue 
is often found upon bugs of the family Belostomidce, espe- 
cially Perthostoma aurantiaca. 
2. A genus of water-beetles, of the family Uy- 
tiscid<B, containing such as the European H. 
tarda. Fabricius, 1801. 
Hydrachnidae (hi-drak'ni-de), n. pi. [NL., < 
Hydrachna + -wte.] A family of aquatic Acari- 
da, typified by the genus Hydrachna ; the water- 
mites. The skeleton is composed of sclerites embedded 
in soft skin, and the body is apparently nnsegmented. 
Most of the Hydrachnidce inhabit fresh water, and many 
are parasitic on mollusks, fishes, and aquatic insects. 
Other genera besides Hydrachna are Atax, Hi/drnchoreu- 
tei, Limnocharei, Pontarachna, and Thalaaarachna, the 
two last named being marine. Also written Hydraraeh- 
nidce. 
hydracidt (hi-dras'id), n. [< hydrtogen) + acid.] 
In chem., a halogen; an acid which does not 
contain oxygen. 
hydracrylic (hi-dra-kril'ik), a. [< hydr(ogen) 
+ acrylic.] Differing from acrylic by the addi- 
tion of the elements of water, HoO Hydra- 
crylio add, C 3 H 6 Os, a monobasic lactic acid which when 
concentrated is a thick non-crystallizable acid syrup, and 
decomposes on heating into water and acrylic acid. 
Hydractinia (hi-drak-tin'i-ii), n. [NL., < Gr. 
Mop (Mp-), water, + Actinia.] The typical ge- 
nus of Hydractiniida;. H. echinata is an example. 
Colonies of these polyps may be found growing on shells, 
forming a delicate white moss-like structure. 
Hydractinlidae (hi-drak-ti-ni'i-de), n. pi [NL., 
< Hydractinia + -idee.'} A family of hydroid 
hydrozoans with free or rudimentary medusae, 
of which the type is the genus Hydractinia. 
These hydroids form polyp colonies consisting of a dense 
mass of hydrorhizse, whence simple or branched hydro- 
caull arise with three kinds of zooids : ordinary nutritive 
zooids with a verticil of filiform tentacles; reproductive 
zooids of each sex ; and a third kind, of slender form, with- 
out tentacles, but bearing cnidocells for the defense of the 
colony. The free medusae have ocelli at the base of the 
tentacles, but no otoliths. The fertilized ova develop into 
planulas. 
Hydradephaga (hi-dra-def'a-ga), n. pi. [NL., 
neut. pi. of *hydradephagus : see hydradepha- 
gous.'} The aquatic and adephagous beetles, 
comprising the two families Dytiscida; and Gy- 
rinidce, in which the legs are fitted for swim- 
ming: distinguished from Geadephaga. Mac- 
leay, 1825. The group is also called Hydrocan- 
thari. 
hydradephagOUS (hl-dra-def 'a-gus), a. [< NL. 
"hydradepliagus, < Gr. Mop (vXp-), water, + 0*7- 
<t>a-yos, gluttonous: see adephagous.'} Aquatic 
and predatory, as certain beetles ; specifically, 
pertaining to or having the characters of the 
Hydradephaga. 
Hydrae, n. Latin plural of Hydra. 
hydraemia, hydraemic. See hydremia, hy- 
dremic. 
hydraform (M'dra-f&rm), a. Same as hydri- 
form. 
h'ydragogic (hl-dra-goj'ik), a. Having the 
character or effect of a hydragogue. 
hydragogue (hi'dra-gog), . [< P. hydragogue, 
< LL. hydragogus, conducting water, a plant so 
called, < Gr. MpayoyAf, conducting water, a wa- 
ter-carrier, an aqueduct, < Mop (Mp-), water, + 
ayuyof, leading, < ayeiv, lead, conduct: see agent.'} 
Inmed. : (a) An active purgative, as jalap, which 
produces a great flux from the intestinal mem- 
Hydralgae (hi-dral'je), n. pi. [NL., < Gr. Mop 
(Mp-), water, + L. alga, seaweed.] Same as 
Hydrophyta. 
hydramnios (hl-dram'ni-os), n. t-NL., < (j r 
Mop (Mp-), water, -f- a/iviav, amnion.] Inpatliol., 
an excessive accumulation of liquor amnii. See 
liquor and amnion. 
Hydrangea (hi-dran'je-a; properly hi-dran- 
je'a), n. [NL., < Gr. Mop (Mp-), water, + dyyeZov, 
vessel: see angio-."] 1. A genus of shrubs or 
herbs, of the natural order Saxifragece, type of 
the tribe Hydrangeas, containing about 33 spe- 
cies, natives of Asia and America, characterized 
by having the ovary inferior, 4 or 5 valvate pet- 
A, hydranthof Athorybia 
rosacea. with a, one of the 
villi; K, a villus in exten- 
sion, enlarged ; C, a small 
retracted villus, still more 
magnified, with its vacuolar 
spaces (*) and cilia. 
Hydrangea arboresfens. n, b, fertile and sterile flowers ; c, fruit cut 
transversely ; d, fruit entire. 
als, 4 or 5 styles, free or connate at base, the fruit 
a capsule, and the leaves deciduous or persis- 
directly from a main (particularly from a main 
in a street), consisting of a hollow cylinder 
provided with one or more nozles to which hose 
may be attached, or with a spout, or the like, 
and usually with a valve and pipe for the es- 
cape of the excess of water, in order to guard 
against freezing. The common form of a fire-hydrant 
is that of an upright pipe standing about two feet above 
the ground, as on the edge of a sidewalk, with a nozle to 
which the filling-hose or suction-pipe of a fire-engine can 
be attached. The valve is below, next to the main, and is so 
arranged that the closing of it opens the waste-pipe and 
frees the hydrant from water. See cut in preceding column, 
hydranth (hi'dranth), n. 
avdof, flower.] Apolypite; 
the fundamental struc- 
tural element in Hydro- 
sou. It consists (with vari- 
ous modifications) of a sac hav- 
ing at one end an ingestive or 
oral aperture leading into a di- 
gestive cavity. The walls of the 
sac are formed of at least two 
cellular membranes, inner and 
outer, or endoderm and ecto- 
derm, which have the morpho- 
logical valence respectively of 
the epithelium and epidermis 
of the higher animals. Between 
these membranes a third layer, 
the mesoderm, may be devel- 
oped. See also cuts under Campamdaria and Diphyulm. 
In an early stage of its existence every hydrozoon is 
represented by a single hydranth, but, in the majority of 
the Hydrozoa, new hydranthx are developed from that first 
formed by a process of gemmation or fission. Huxley. 
drapult (hi'dra-pult ), n. See hydropult. 
drarchus (hl-drar'kus), . [NL., < Gr. Mop 
water, + ap%6<;, ruler, < ap^eiv, rule.] 
A genus of fossil cetaceans : same as Basilo- 
saurus. Also Hydrarclios. Koch. 
hydrargillite (hi-drar'ji-llt), n. [< Gr. Mop 
(Mp-), water, + dpy</!/lof, white clay : see argil- 
laceous.'} A crystalline variety of gibbsite, a 
hydrous oxid of aluminium, 
hydrargiret, . See hydrargyre. 
hydrargochlorid, hydrargochloride (hi-drar- 
go-klo 'rid, -rid or -rid), n. [< hydrarg(yrum) 
+ chlorid.] A double chlorid of mercury and 
some other base: as, hydrargochlorid of am- 
monium. 
hydrargyralt, a. [< hydrargyrum + -al] Mer- 
curial. Bailey. 
hydrargyrate (hi-drar'ji-rat), a. [< hydrargy- 
rum + -atei.} Of or pertaining to mercury, 
hydrargyret, n. [= Sp. hidrargirio =Pg. hydrar- 
gyro = It. idrargiro; < L. hydrargyrus, quick- 
silver: see hydrargyrum.] Quicksilver; mer- 
cury. Also spelled hydrargire. 
Th' hidden loue that now-a-dayes doth holde 
The Steel and Loadstone, Hydrargire and Oolde, 
Th' Amber and straw. 
Sylvester, tr. of Du Bartas's Weeks, ii., The Furies. 
hydrargyria (hi-drar-jir'i-a), n. [NL., < hy- 
drargyrum.'} Same as hydrargyriasis. 
hydrargyriasis (hi-drar-ji-ri'a-sis), >. [NL., 
tent The common hydrangea, //. Hortemia, is a native < hydrargyrum + -iasis.} In pathol., mercurial 
of China. It was introduced into England by Sir J. Banks poisoning ; a morbid condition produced by the 
introduction of mercury into the animal sys- 
tem. Also called hydrargyrism, hydrargyrosix, 
hydrargyria, hydrargysm. 
in 1790. It is a favorite for the beauty and size of its flow- 
era, which form immense globular clusters, blue, pink, or 
white. H. arborescens, the wild American hydrangea, was 
introduced into European cultivation from Virginia in . -, ~~ , , ~.. 
1738 ; it is not much cultivated in the United States. H. hydrargyriC (hi-drar-jir'ik), a. [< hydrargyrum 
quercifolia, the oak-leafed hydrangea, is wild from Georgia + -ic.] Pertaining to hydrargyrum, or mer- 
south, and is hardy at the north in cultivation. The ge- curv" mercurial 
been 1 des cri 1 bed n from the Miocene"!}? ESope^S^mettaTef hydrargyrism (hi-drar'ji-rizm), . [< hydrar- 
rlled Hydranyia. gyrum + -MM.] Same as hydrargyriasis. 
[I. c.] A plant of this genus. 
hydrangead (hi-dran'je-ad), n. [< Hydrangea 
+ -ad l 7\ A plant belonging to Lindley's order 
Hydrangeacea-, now placed in the order Saxi- 
fragece, tribe 
Hydrangea 1 . 
(hi-dran'je-e), 
n.pl. [NL.'(A. 
P.deCandolle, 
1830), < Hy- 
drangea + 
-e<K.~} Atribeof 
plants of the 
natural order 
Saxifrages. 
They are shrubs 
or trees with op- 
posite exstipulate 
leaves, petals of- 
ten valvate, sta- 
mens often epi- 
gynous, and the 
ovary in most of 
the genera 3- to 
5-celled. 
Street Fire-hydrants. 
hydrargyrum (hi-drar'ji-rum), . [NL., < L. 
hydrargyrus, < Gr. Mpapyvpof, quicksilver (as ar- 
tificially prepared from cinnabar ore; native 
quicksilver was called apyvpos xm6<;, ' fused sil- 
ver'), < Mop (Mp-), water, + apyvpof, silver: 
see argent.'} Chemical symbol, Hg. Quicksil- 
ver; mercury. See mercury. 
hydrargysm (hi-drar'jizm), . Same as hy- 
drargyriasis. 
hydrarthrosis (hi-drar-thro'sis), n. [NL., < Gr. 
Mop (Mp-), water, + ap6poatf, a jointing: see 
arthrosis."] In pathol., the accumulation of se- 
rous liquid in a joint-cavity. 
hydrarthrus (hi-drar'thrus), . [NL., < Gr. 
Mop (Mp-), water, + apOpov, joint.] Same as 
hydrarthrosis. 
HydraspidaB (hi-dras'pi-de), . pi. [NL., < Hy- 
draspis + -ida;.] Same as Hydraspididai. 
Hydraspididifi (hi-dras-pid'i-de), n. pi. [NL., < 
Hydraspis (-id-) + -ida;.] A family of tortoises, 
typified by the genus Hydraspis. The head is 
depressed and covered with small polygonal plates, and 
the flat skull has a distinct bony crown with a more or 
less elevated occipital arch. The species inhabit South 
America and Australia. In Cope's system of classification 
