Notwithstanding the philosophic serenity which Bennet recommends, the fact of new individuals resulting from dividing these fresh-water polyps was always a subject of profound astonishment, and of never-ending meditation. SERTULARIADÆ. All Hydraidæ, with the exception of the Hydra and a few other genera, are marine productions, varying from a few lines to upwards of a foot in height, attaching themselves to rocks, shells, sea-weeds, and corallines, and to various species of shell-fish. Many of them attach themselves indiscriminately to the nearest object, but others show a decided preference. _Thuiaria thrya_ attaches itself to old bivalves; _Thoa halecuia_ prefers the larger univalves; _Antennularia antennina_ attaches itself to coarse sand on rocks; _Laomedea geniculata_ delights in the broad frond of the tangle; _Plumularia catherina_ attaches itself in deep water to old shells, corallines, and ascidians, growing in a manner calculated to puzzle the naturalist, as it did Crabbe, the poet, who writes of it:-- "Involved in sea-wrack, here you find a race Which science, doubting, knows not where to place; On shell or stone is dropp'd the embryo seed, And quickly vegetates a vital breed." _Sertularia pumila_, on the other hand, loves the commoner and coarser wracks. "The choice," says Dr. Johnston, "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 deep seas must select between rocks, corallines, or shells." There seems to be a selection even as to the position on the rocks. According to Lamouroux, some polyps always occupy the southern slopes, and never that towards the east, west, or north; others, on the contrary, grow only on these exposures, and never on the south, altering their position, however, according to the latitude, and its relation to the Equator. The _Sertulariadæ_ have a horny stem, sometimes simple, sometimes so branching that they might readily enough be mistaken for small plants, their branches being flexible, semi-transparent, and yellow. Their name is derived from _Sertum_, a bouquet. Each Sertularia has seven, eight, twelve, or twenty small panicles, each containing as many as five hundred animalcules; thus forming, sometimes, an association of ten thousand polyps. "Each plume," says Mr. Lister, in reference to a specimen of _Plumularia cristata_, "might comprise from four to five hundred polyps;" "and a specimen of no unusual size now before me," says Dr. Johnston, "with certainly not fewer cells on each than the larger number mentioned, thus giving six thousand as the tenantry of a single polypidom, and this on a small species." On _Sertularia argentea_, it is asserted, polyps are found on which there exist not less than eighty to a hundred thousand.