The Bread-crumb sponge (_Halichondria panicea_) is even more common, for it is to be found on every rocky coast, encrusting weeds and rocks, often considerably above low-water mark. It is of a yellowish or pale greenish colour, and forms an incrustation varying in thickness from one-twentieth of an inch to half an inch or more; and, like most sponges, should be looked for in narrow crevices, under heavy growths of weeds, or in other situations where it is protected from the light. Sometimes its free surface is unbroken, except, of course, by the minute pores, and, here and there, the larger openings that serve for the outgoing currents; but when it is found encrusting a rock in patches of considerable size, the larger holes all occupy the summit of a little cone resembling a miniature volcano with its crater. This sponge is easily removed from the rock with the aid of a blunt broad-bladed knife, and retains its natural appearance to perfection if preserved in methylated spirit. Its horny skeleton is of a very compact nature, and the spicules are minute siliceous needles pointed at both ends. Rambling on the sea beach we frequently meet with old oyster and other shells perforated by a number of circular holes about the size of a pin’s head or less, and chalk and limestone rocks also are seen similarly bored. On breaking into or grinding down the substance we find that the openings are the ends of channels that form a network of canals and chambers, some of which are so near the surface that they are covered by an exceedingly thin layer of the calcareous substance. These canals and chambers form the home of the Boring Sponge (_Cliona_), which, although a very soft-bodied animal, has itself excavated them. [Illustration: FIG. 74.--SPICULES OF _Halichondria_, MAGNIFIED] [Illustration: FIG. 75.--AN OYSTER SHELL BORED BY _Cliona_] The manner in which the _Cliona_ excavates such a complicated system of passages in so hard a material has naturally raised a considerable amount of curiosity, and those who have studied the matter are divided in opinion as to whether the work is done by chemical or by mechanical action.