Curiously enough, these old sponges did not avail themselves of the natural cystallization of silica, which, left to itself, would have formed six-rayed stars, with the rays at angles of sixty degrees, or six-sided plates, rods, or pyramids. They adopted another and peculiar form of the mineral, known as colloidal silica, and being thus relieved from any need to be guided by its crystalline form, treated it as we do glass, and shaped it into cylindrical tubes, round needles and stars or crosses, with the rays at right angles to each other. The sponges whose skeletons are thus constructed, and which anticipated so many mechanical contrivances long afterwards devised by man, belonged to a group of silicious sponges (_Hexaclinellidæ_) which is still extant, and represented by many rare and beautiful species of the deep sea, which are the ornaments of our museums, and of which the beautiful Eupleectella or Venus flower-basket, from the Philippine Islands, and the glass-rope sponge (_Hyalonema_), from Japan, are examples. But contemporary with these there was another group (_Lithistidæ_), constructing skeletons of carbonate of lime, and which preferred, instead of the regular mechanical structures of the others, a kind of rustic work, made up of irregular fibres, very beautiful and strong, but as a matter of pattern and taste standing quite by itself. If there were any sponges with altogether soft and spongy skeletons in these old times, their remains do not seem to have been preserved. Here, it will be observed, are a great variety of vital and mechanical contrivances devised in the very early history of the earth, settled for all time, and handed down without improvement, and with little change, to our later day. They are indeed vastly more wonderful than the above general account can show; for to go into the details of structure of any one of the species would develop a multitude of minor complexities and niceties which no one not specially a student of these animals could appreciate. These are not solitary cases. The student of fossils meets with them at every turn; and if he possesses the taste and imagination of a true naturalist, cannot fail to be impressed with them.