82 ZOOPHYTA HYDKOIDA. 



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 veofetation 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, a.) 

 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."— Corall. Flex. Introd. p. L. 



