PLATE I.
ZOOPHYTA. POLYZOA.
THE forms of animal life which are now united in an independent class,
under the name Polyzoa, so nearly resemble the Hydroid Zoophytes in
general form and appearance that a casual observer may suppose them to be
nearly identical. In all but the more recent works, they are treated as
distinct indeed, but still included under the general term “ZOOPHYTES.”
The animals of both groups are minute, polypiform creatures, mostly
living in transparent cells, springing from the sides of a stem which
unites a number of individuals in one common life, and grows in a
shrub-like form upon any submarine body, such as a shell, a rock, a weed,
or even another polypidom to which it is parasitically attached. Each
polype, in both classes, protrudes from and retreats within its cell by
an independent action, and when protruded puts forth a circle of
tentacles whose motion round the mouth is the means of securing
nourishment. There are, however, peculiarities in the structure of the
Polyzoa which seem to remove them from Zoophytology to a place in the
system of nature more nearly connected with Molluscan types. Some of
them come so near to the compound ascidians that they have been termed,
as an order, “Zoophyta ascidioida.”
The simplest form of polype is that of a fleshy bag open at one end,
surmounted by a circle of contractile threads or fingers called
tentacles. The plate shows, on a very minute scale, at figs. 1, 3, and
6, several of these little polypiform bodies protruding from their cells.
But the Hydra or Fresh-water Polype has no cell, and is quite unconnected
with any root thread, or with other individuals of the same species. It
is perfectly free, and so simple in its structure, that when the sac
which forms its body is turned inside out it will continue to perform the
functions of life as before. The greater part, however, of these
Hydraform Polypes, although equally simple as individuals, are connected
in a compound life by means of their variously formed _polypidom_, as the
branched system of cells is termed. The Hydroid Zoophytes are
represented in the first plate by the following examples.