The annexed figure (Fig. 16) shows their general appearance and structure. The lower end was fixed to the rock like the flat sucker-like disc of the actinia. Around the outer margin there diverged one or more rows of slim tentacles, hollow, soft, and retractile, like those of the actinia. From the margin to the centre there radiated more than a hundred lamellæ, but these differed from the corresponding membranes of the modern animal, inasmuch as they were strengthened internally by a skeleton of hard carbonate of lime; and to this difference we owe their preservation. They stand out in high relief upon weathered specimens, showing the long, narrow chambers that ran between them. Their walls were once doubtless hung with countless vibratile cilia, and perhaps pierced each with its window, through which the currents of water passed in their ceaseless progress to and from the sea. At the centre lay the mouth, communicating by a short gullet with the stomach, which occupied the central portion of the animal, and from the outer walls of which the lamellæ diverged like so many buttresses. In its youngest stages, the animal occupied the whole length of the cup, but, us it increased in size, it gradually retreated from the narrow end, which was then divided off by a thin calcareous membrane. At each successive stage of its growth, a new membrane was added, each further and further from the lower end, so that eventually the creature left below it a series of empty chambers all firmly built up. Thus, in a specimen six or eight inches long, there would in reality only be a small part tenanted--in fact merely the upper floor--all the lower storeys remaining silent and uninhabited. The house of this old-world architect differed widely in one respect from human dwellings. Man begins his basement story of the same dimensions as those that are to succeed it, or, if any difference is made at all, the upper floors are built each less than the one below it, so that the whole structure tapers upward to a point, as in the Pyramids. But the cyathopsis reversed this latter process; it inverted the cone, commencing the smallest chamber at the bottom, and placing the widest at the top. Indeed, one is sometimes puzzled to conjecture how so bulky a building could be securely poised on so narrow a basis, and it is certainly difficult to see how the creature could move about with such a ponderous load to drag along. The snail carries his house on his back, yet it is a slim structure at the best; but the cup-coral must not merely have carried his house, but some dozen or two of old ones strung one after another to his tail. Perhaps, though free to move about and try change of residence in its youthful days, the creature gradually settled down in life, and took up its permanent abode in some favourite retreat, the more especially as in process of time it became what we should call a very respectable householder.