ECHINODERMATA. 623 ECHTEB. that its mfiiilwis are characteristic index fos- sils of tlio Carboniferous and I'erniian rocks. The class l>ecanie extinct at the close of the Paleo- zoic. The body is inclosed in a bud formed box n:ade up;.;enerally of thirteen plates of regular form and strictly pentaujeral radial arrange- ment. The mouth is at the summit of the calyx, and the anus at the side. There are no arms, and the relatively large ambulacra! areas extend downward over the surface of the calj-.x, some- times nearly to the base or stem. The stem was short or absent. About 20 genera and 150 spe- cies are known. See Pe.ntremites. Tor description of the ontogeny or develop- ment of the echinodcrms, consult the works by Lang and Bather mentioned in the bibliography. The following diagram is intended to represent both tile geological distribution and the phylo- genetic relationships of the various classes: 1 -o E to g i J .3 E ^ i 3 5 1 c 1 1 Q- 1 a: OphiuraifJea Astero'dea — ^ __ ^
,■■ " BUstnidea — 2 ■ ■ J L — — ^^ 51 m ■ — r — — Exhinoidca ■^ — «■ ■ ■ ■ HolothuroideB
^
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rn
BlBLiocRAPiiY. For general treatises on both
living and fossil forms, see Bather, "The Echi-
noderma." in Lankester's Treatise on Zoology,
part ii. (London, 1900); this contains a good
I'ililiography. Consult, also: Parker and Has-
well, Textbook of Zoology, vol. i. (London and
Xew York, 1897); Lang and Bernard. Text-Book
of Comparatire Anatomy, part ii. (London and
Xew York, 1896). also with a bibliogi-aphy.
These last two works are technical. Zittel and
Eastman, Text-Rook of Paleontology, vol. i.
(London and Xew York, 1900), deals almost
entirely with the fossil forms. See, also, Alex-
ander Agassiz, "Paleontological and Embryo-
logical Development," Proeeedings of the Ameri-
can Association for the Aelraiicemeiit of Science,
Boston Meeting (Cambridge. 1880): Carpenter,
"Notes on Echinoderm Morphology," Quarterly
Journal of the Microscopical Society, new series,
vols, xviii.-sxviii. (London, 1878-87) : Bell. "On
the Arrangements and Interrelations of the Class-
es of the Echinodermata," Annals and Magazine
of Xatural History, series vL, vol. vii. (London,
1891); Ludwig, Morphologische Stiidien an
Kchinodermen (Leipzig, 1877-82); Xeumayr. Die
Stiimme des Thicrreichcs (Vienna and Pragiie.
1SS9). See, also, the works cited under the
articles on various classes of echinodcrms above
referred to.
.ECHINOIDEA. ek'i-noi'dA a (X'eo-Lat. nom.
pi., from Ok. ^x'""'- cchinns. hedgehog + elSos,
eidos. form). One of the five classes of Echino-
dermata (q.v.). containing the sea-urchins and
their kindred. It is sulidivlded into four orders:
(1) Palajo-Echinoidea, all fossil; (2) Regularia,
the typical globular echinoids or sea-urchins;
(3) Clypeastridea, the llattened cake-urchins
and sand-dollars: (4) Spatangoidea, the cordate
heart-urchins. For further details of existi;ig
fuiiMS, see SK.-fBCinx.
ECHIUM, ek'i-nm. See Viper's Btgloss.
ECHIURIDA, ek'i-u'rlda. An order of anne-
lids. See An.NVL.TA.
ECHMIADZIN, 6ch'me-ad-zen'. See Eicii-
MIAll/lX.
ECHO (Lat., from Gk. 'IIx*, (rom-iixor, cchos,
sound). A personification in Greek mythology.
Acconling to the legend, the nymph Echo, by
her talking, detained Hera when .searching for
Zeus among the mountain nymphs. .Vs a punish-
ment the goddess deprived her of the power of
speech unless first spoken to, when she was
compelled to rejdy. The love of Pan for Echo
was also celebrated by later poets, .ccording
to Ovid, the nyinph Echo, having conceived a
hopeless passion for Xareissus, pined away until
only her voice remained. A portico at Olympia,
where there was a seven-fold echo, was called by
the nymph's name.
ECHO. .Sound is produced by longitudinal
waves or vibrations of the air or other medium
through which it travels, and these waves on
striking a surface are reflected according to the
laws of reflection. A sound thus reflected is
termed an echo, and the rule that the angle of
incidence nuist equal the angle of reflection
holds good, as in the case of light and other
forms of wave-motion. If the sound is reflected
by an obstacle at right angles to the direction
of propagation, the echo returns to an obsen-er
at the place where the soimd originated. This
phenomenon enables one to ascertain the dis-
tance to a clifT or other surface from which an
echo is obtained, and is made use of by naviga-
tors when approaching, in a fog. a shore with
cliffs. Knowing that sound at ordinary tempera-
tures travels with an apprn.ximate velocity of
1125 feet per second, by multiplying this figure
by the time between the sound — that of a steam
whistle for example — and the perception of the
echo, we obtain the distance to the cliff and
back. Reflection also takes place from the surface
of a cloud, as is the case with thunder, and an
uneven surf;icc .such as that iiresented from the
edge of a wood, which often will furnish excellent
echoes.
ECHO, IN Mrsic. (1) A subdued repetition
of a phrase. (2) In large organs there is a
separate set of pipes, placed at a distance, so
as to produce the effect of an echo. There are
separate stops and generally also a special
manual.
ECHO CANON. . long and deep gorge in
northern Ltah, on the Union Pacific Railroad
(Map: Utah, B 1). It is aliout 25 miles north-
east of Salt Lake, and is remarkable for its
echoes and sublime .scenery. The sides of the
canon are of bare rock and almost vertical.
Owing to erosion, many of the rocks have as-
sumed curious shapes.
ECHTER, C-K'ler, MirH.Ei. (1812-70). A Ger-
man painter. He was born in 'Munich, received
his artistic training at the academy (here, and
assisted Julius Schnorr von Karolsfcld (q.v.)
in the dei'oration of the sallc dcs files of the
royal palace at ilunich with nineteen wall paint-
