Encyclopedia Britannica 1963 [12].pdf

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THE
UNIVERSITY
OF
CHICAGO
The Encyclopadid Britannica
is
published with the editorid advice ofthe faculties
of
The University
of
Chicago and of a
committee of members of thefdrzlltiPJ of Oxford,,Cambridge
and London unive~ersitiesand of a committee
of
at The Unit'e~sity Toronto
t
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O
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O
RE
AND THUS BE HUMAN LIFE
E
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ED
."
A
New Sa~vey U ~ i z : e K~owledge
of
~~d
Volume
12
HYDROZOA
EMY
,
E
PISTLE
OF
T
O
J
ER
ENCYCLOPEDIA BRITANNICA, INC.
W I L L I A M
BENTON,
P U B L I S H E R
CHICAGO. LONDON
TORONTO
-
GENEVA
SYDNEY
PRINTED IN THE
U.
S .
A .
F
O
UN
DED
A.D.
1768
Volume
1 2
The Hydrozoa (sometimes called
Hydromedusae) are a class of animals, the vast
majority of which are marine, and which belong
to the still greater assemblage knomn as the Coel-
enterata
(9.v.).
The Hydrozoa include not only
polyps, but also medusae or jellyfish (these terms are de-
They are, in fact, that
fined in the article COELENTERATA).
group of Coelenterata in which neither the one nor the other of
these two forms of body predominates, and in this respect they
contrast strongly with the other main classes (Scyphozoa and
Anthozoa). Moreover, both polyp and medusa have a simpler
plan of structure than in the other classes. The polyp itself is
frequently (though not always) small. Its mouth leads directly
into the internal cavity of its body (coelenteron), without the
intermediary of a definite throat or gullet of any kind, and the
ectoderm and endoderm
(see
COELENTERATA) at the lips.
meet
The coelenteron is a simple cavity lined by endoderm; it is not
subdivided by partitions into lesser cavities, nor does it contain
definite organs of any kind. With these limitations the actual
form of the polyp varies very greatly. The medusa presents in-
finite variety of form, but it too lacks a throat, and although its
coelenteron sends out radiating canals which run from the central
cavity through the solid tissues of the bell, it is otherwise simple
in that it contains no definite organs. The medusae of Hydrozoa
are, generally speaking, smaller and more slightly built creatures
than the medusae belonging to the related class, Scyphozoa,
al-
though in certain cases they attain a larger size than the average,
which is a matter of millimetres. The Hydrozoa are also charac-
terized by the fact that the sex cells, when they ripen in the clus-
ters known as
gonads,
typically lie in or under the ectoderm. al-
though the site of their original formation may be in either
ectoderm or endoderm. I t is among the Hydrozoa above all other
Coelenterata that the phenomenon, briefly characterized else-
where (article COELEXTERATA), known as
polymorphism,
at-
and
tains its height. The details of this condition are described in
parts of the present article and a summary of the question is
given after the section on Siphonophora.
The infrequency of brackish or fresh-water forms among the
Coelenterata makes their occurrence of interest. The ordinary
marine Hydrozoa are either pelagic (swimming or floating organ-
YDROZOA.
isms) or sedentary, according to their nature, and many of either
kind exist. The brackish and fresh-water forms exhibit the same
diversity, though few in number. One of the most interesting is
a minute creature,
Protohydra,
the length of which is about
3
mm.
This organism inhabits the surface layer of mud, rich in diatoms,
which is to be found in the bottom of pools in certain tidal
marshes: it also occurs in oyster beds and similar places. I t is
carnivorous, and reproduces freely by transverse fiss~on. I t
pas-
sesses no tentacles, and is as simple in structure as any known
Coelenterate.
The best known of the nonmarine Hydrozoa, however, are
the genera
Li?nnocodiu.nz, Limnocnida, Cordylophora
and
Hydra.
Of
Hydra
more details are given below, and the chief interest of
CordyloPhora
lies in the fact that it may flourish in water of
different degrees of salinity as well as in fresh water; i t is other-
wise ordinary.
Li?nnocodium ryderi
possesses a feebly developed
polyp generation (up to about
2
mm. long) u7hich produces small
colollies containing about
2-7
individual polyps w'ithout tentacles.
These colonies can produce buds of two kinds; some become sep-
arated from the parent, form polyps and produce new colonies,
others develop into medusae, and these are liberated and swim
away. The species of
Limnocodiz~m
(with which is now included
M z c r o h ~ d r a )
are not very clearly recognized; but representatives
of the genus occur in lakes, millstreams and similar places in the
United States, Germany, China and Japan, and have appeared
in water lily tanks at various botanic gardens, and in other tanks
and aquaria. The genus
Limnocnida
contains medusae which have
been found in several r i ~ e r
systems in Africa and in some of the
great lakes, as well as in India. The Hydrozoa comprise three
large orders which from this point will be treated separately.
ORDER I, HYDROIDA
The Hydroida are, roughly, those Hydrozoa which possess a defi-
nite alternation of the polyp and the medusa in their life history,
and in which one generation (the polyp) is sedentary and usually
constructs a fixed colony, the other being free-swimming when fully
developed. There are exceptions to this general statement, but they
are not characteristic of the group as a whole. The variety of form
ho~vever,is
great The common and well-
and life history exh~bited.
known but quite untypical fresh-water genus
Hydra
(tig.
I
) ,

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