Whoever has spent a few days on some rocky coast, must have noticed adhering to half-tide stones numerous solitary _actiniæ_. Arrayed in all the colours of the rainbow--purple, green, and gold--these little creatures hang out their tentacles like so many flowers, and have hence received the popular name of sea-anemones. Their internal structure is no less beautiful. They resemble so many large plump gooseberries, and consist of a little sack suspended within a larger one. The outer sack is fringed along its upper edges with one or more rows of slim hollow tentacles, which diverge outwards like the petals of the daisy, and can be contracted at pleasure so as somewhat to resemble the daisy when folded up at sunset. The inner sack, which forms the stomach of the animal, has a short opening or gullet, at the upper part of which is the mouth lying in the centre of the cavity surrounded by the fringes of tentacles. The inner sack is connected with the outer by means of thin membranes, like so many partition-walls, which radiate inwards like spokes towards the axle of a wheel. The space between each of these membranes, or lamellæ, forms an independent chamber, but it has a communication with those on either side by a window in each wall, and further opens upwards into the hollow tentacles, which, with minute orifices at their outer points, may be compared to chimneys. These chambers form the breathing apparatus of the little creature. Sea-water passes down through the tentacle into the hollow chamber below, whence, by the constant action of minute hairlike cilia that line the walls like tapestry, it is driven through the window into the next chamber, thence into the next, and so on, passing gradually through the tentacles back to the sea. The actiniæ are of a soft perishable substance, but many of the other _Anthozoa_, or flower-like animals, have hard calcareous skeletons. Of such a kind are the polypi that in the Pacific Ocean have raised those stupendous reefs and islands of coral. It does not appear that, during the Carboniferous period, there existed any reef-building zoophytes, but some of the most abundant forms of life belonged to a kindred tribe, and are known by the name of _Cyathophyllidæ_, or cup-corals. As the name imports, the typical genus has a general cup-shaped form, but this is liable to many aberrations in the cognate genera. The younger specimens of one species (_Cyathopsis fungites_) have a curved outline somewhat like the bowl of a tobacco-pipe, whence the quarrymen know them as pipe-heads. The older individuals are generally more or less wrinkled and twisted, sometimes reaching a length of eight or nine inches, and have been named by the workmen _rams'-horns_. [Illustration: Fig. 16.--Cyathopsis (clisiophyllum ?) fungites.]