Generally this fibrous mass is interwoven with numerous mineral spicules of a wonderful elegance and variety of forms, for their shapes are not only strictly determinate for each species of sponge but each part of the sponge, it is believed, has spiculæ of a character peculiar to itself. Sometimes they are pointed at both ends, sometimes at one only, or one or both ends may be furnished with a head like that of a pin, or may carry three or more diverging points, which sometimes curve back so as to form hooks. Sometimes they are triradiate, sometimes stellar; in some cases smooth, in others beset with smaller spinous projections like the lance of the saw-fish. In many species they are embedded in the horny framework; in others, as, for instance, in Tethea Cranium, or in Halichondria, they project from its surface like a tiny forest of spears. They are generally composed of silex or flint, but in the genus Grantia they consist of carbonate of lime. Though the skeleton of most sponges is formed both of horny fibres and of mineral spicules yet the proportions of these two component parts vary considerably in different species. In the common sponge, for instance, the fibrous skeleton is almost entirely destitute of spicules, a circumstance to which it owes the flexibility and softness that render it so useful to man, while they predominate in the Halichondriæ, and sometimes even, as in the Grantiæ, completely supersede the horny fabric. [Illustration: Minute portion of the surface of Tethea Cranium, magnified, spicula projecting beyond the surface.] [Illustration: Halina papillaris. Currents passing inwards through the pores (_a a_), traversing the internal canals (b), and escaping by the larger vents (_c_, _d_).] On examining a sponge, the holes with which the substance is everywhere pierced may be seen to be of two kinds; one of larger size than the rest, few in number, and opening into wide channels and tunnels which pierce the sponge through its centre; the other minute, extremely numerous, covering the wide surface, and communicating with the innumerable branching passages which make up the body of the skeleton. Through the smaller openings or pores the circumambient water freely enters the body of the sponge, passes through the smaller canals, and, ultimately reaching the larger set of vessels, is evolved through the larger apertures or oscula. Thus by a still mysterious agency (for the presence of cilia has as yet been detected but in one genus of full-grown marine sponges) a constant circulation is kept up, providing the sponge with nourishing particles and oxygen, and enabling its system of channels to perform the functions both of an alimentary tube and a respiratory apparatus.