In describing sponge as an animal substance, it may be necessary to explain that the sponge in ordinary use is the flexible skeleton of a living zoophyte, and was originally invested with a gelatinous or slimy matter, which lined all the pores and channels. When alive in the water, currents constantly enter the outer pores, traverse all the internal inosculating canals, and issue from the larger orifices which often project above the surface in perforated papillæ. By the circulation of the sea-water through the porous structure, the nutrition of the animated mass is effected; and the modifications observable in the number, size, form, and arrangement of the pores, canals, and apertures, in the different kinds of this type of organization, are subservient to this especial function. But associated with the true _Poriferæ_ or sponges, are numerous zoophytes which resemble them in form, but are of an entirely distinct nature; for they are the fossilized remains of _Polyparia_, that is, of the frame-work of an aggregation of polypes, each individual of which had an independent existence, although the whole were united by one common living integument, like the _Alcyonium_, or dead-men's fingers, of our coasts.[AP] [AP] See '_Medals of Creation_,' p. 251. [Illustration: Lign. 25:--Flints deriving their forms from the zoophytes they enclose.] [Sidenote: FUNGIFORM FLINTS.] Among the flints whose forms depend on the organic bodies they enclose, are some which bear so close a resemblance in shape to _Fungi_, that they are provincially called in Sussex "_petrified mushrooms_;" several of them are figured above (_Lign. 25_). In these fossils there are openings at the base, and a groove on the margin of the upper part, in which the structure of the enclosed body is generally more or less distinctly seen; and upon breaking one of these bodies, a section of a funnel-shaped zoophyte is obtained. The origin of these flints will be understood by reference to the four interesting specimens here delineated, one-sixth of the natural size in linear dimensions. [Illustration: Lign. 26:--Ventriculites from the Chalk, Lewes. 1. A perfect specimen in Chalk, shewing the external net-like surface. 2. An expanded specimen, displaying the inner surface studded with cells. 3. A Ventriculite with the lower part enveloped in Flint. 4. Part of a Ventriculite; the base invested with Flint: the root-like fibres are seen at a. ] [Sidenote: VENTRICULITES.]