82 
ZOOPHYTA HYDROIDA. 
height. They are all, with the exception of the hydra or fresh- 
water polype, marine productions, and are found attached to 
rocks, shells, sea- weed, other corallines, and to various shell-fish. 
Many of them appear to be indiscriminate in their choice of the 
object, but others again make a decided preference. Thus 
Thuiaria thuja prefers the valves of old shells, Thoa helicina is 
more partial to the larger univalves, Antennularia antennina 
grows on rocks, Campanularia genieulata delights to cover the 
broad frond of the tangle with a fairy forest peopled with its 
myriads of busy polypes, while the Sertularia pumila rather loves 
the more common and coarser wracks. The choice may in part 
be dependent on their habits, for such as are destined to live in 
shallow water, or on a shore exposed by the reflux of every tide, 
are in general vegetable parasites ; while the species which 
spring up in the deep seas must select between rocks, corallines 
or shells, the depths at which they are found being too great for 
the vegetation of sea-weed.* 
The polypidoms are confervoid and more or less divided, the 
ramifications being disposed in a variety of elegant plant-like 
forms. The stem and branches are alike in texture, slender, 
horny, fistular, and almost always jointed at short and regular 
intervals, the joint being a mere break in the continuity of the 
sheath without any character of a proper hinge, and evidently 
formed by regular periodical interruptions in the growth of the 
polypidoms. Along their sides, or at the extremities, we find 
the denticles or cup-like cells of the polypes arranged in a de- 
terminate order, either sessile or elevated on a stalk, (Fig. 9, «•) 
Though of the same substance, the cell is something more than 
a simple expansion of the stem or branch, for near its base there 
is a distinct partition or diaphragm on which the body of the 
polype rests, with a plain or tubulous perforation in the centre, 
through which the connection between the individual polype and 
* Lamouroux says , — “ We find some polypidoms placed always on the south- 
ern slopes of rocks and never on that towards the east, west, or north. Others, 
on the contrary, grow only on these exposures, and never on the south. Some- 
times their position is varied according to latitude, and the shores inclined to- 
wards the south, in temperate or cold countries, produce the same species as the 
northern exposures in equatorial regions ; in general their branches appear di- 
rected towards the main sea.” — Corail. Flex. Introd. p. L. 
