38 
GUIDE TO THE CORAL GALLERY. 
HYDROZOA. 
[(*) An asterisk against names of species denotes that specimens in fluid 
are exhibited in the upright part of Table Case 3.] 
Introduction. 
The exhibited collection of Hydrozoa occupies Cases 2-4 at the 
eastern end of the Coral Gallery. 
The members of the class occur as fixed plant-like forms often of 
horny texture (, Sertularia , &c., Case 3), or as massive or branched 
coral-like growths (Stylaster and Millepora, Cases 2, 4), or as 
transparent free-swimming bell- or disk-shaped organisms, which 
may be simple ( Medusa or Jelly-Fish, Case 3), or may form colonies 
composed of variously modified individuals ( Siphonophora , Case 3). 
The vast majority of species are marine, but a few live in fresh 
water. 
The specimens in Case 3 A, B, closely resemble dried seaweeds, but 
can generally be distinguished from plants by observing, especially 
with a lens, that a serrated appearance of the branches is due to 
little horny cups or receptacles ; see, for instance, Diphasia tamarisca* 
which has unusually large cylindrical cups. In life, each cup 
contains a polyp with a crown of tentacles surrounding a mouth 
opening into a stomach cavity. 
In spite of great differences in form amongst the Hydrozoa, a 
comparatively simple plan of organisation can be traced in all. 
To briefly explain this, an account is given of Hydra or the 
Fresh-Water Polyp, a tiny Hydrozoon which lives in ponds attached 
to water-weeds (Fig. 1a). Hydra , which is green or reddish-brown 
in colour, according to the species, attains an average height of 
about one-third of an inch. The little creature alters its shape 
considerably, being now contracted down to a lump, now expanded 
into a little column with a circle of thread-like tentacles near the 
summit. When a tentacle touches some small organism, the latter 
is paralysed and drawn into the mouth at the top of the column, 
and thence into the simple stomach-cavity, where it is digested, the 
remains being evacuated by the mouth. Between the mouth and 
