The classes of cellular elements in the sponge are as follows: Flattened cells termed "pinacocytes" cover all the free surfaces, that is to say, the external surface and the walls of the {171}excurrent and incurrent canals. The flagellated chambers are lined by "choanocytes" (cf. Fig. 70, p. 176); these are cells provided at their inner end with a flagellum and a collar surrounding it. They resemble individuals of the Protozoan sub-class Choanoflagellata, and the likeness is the more remarkable because no other organisms are known to possess such cells. Taken together the choanocytes constitute the "gastral layer," and they are the active elements in producing the current. The tissue surrounding the chambers thus lying between the excurrent and incurrent canals consists of a gelatinous matrix colonised by cells drawn from two distinct sources. In the first place, it contains cells which have a common origin with the pinacocytes, and which together with them make up the "dermal layer"; these are the "collencytes" and "scleroblasts"; secondly, it contains "archaeocytes," cells of independent origin. Collencytes are cells with clear protoplasm and thread-like pseudopodial processes; they are distinguished as stellate or bipolar, according as these processes are many or only two. Scleroblasts or spicule cells are at first rounded, but become elongated with the growth of the spicule they secrete, and when fully grown are consequently fusiform. [Illustration: FIG. 65.—Diagrammatic section of a siliceous Sponge. _a.p_, Apopyle; _d.o_, dermal ostia; _ex.c_, excurrent, or exhalant canal; _in.c_, incurrent canal; _o_, osculum. (Modified from Wilson.)] Each spicule consists of an organic filamentar axis or axial fibre around which sheaths of silica hydrate are deposited successively by the scleroblast. Over the greater length of the spicule the sheaths are cylindrical, but at each end they taper to a point. The axial canal in which the axial fibre lies is open at both ends, and the fibre is continuous at these two points with an organic sheath, which invests the entire spicule. From this structure we may conclude that the spicule grows at both ends—_i.e._ it grows in two opposite directions along one line—it has two rays lying in one axis, and is classed among uniaxial diactinal spicules. Being {172}pointed at both ends it receives the special name _oxea_. The lamination of the spicule is rendered much more distinct by heating or treatment with caustic potash.[197] [Illustration: FIG. 66.—Cut end of a length of a siliceous spicule from _Hyalonema sieboldii_, with the lamellar structure revealed by solution. × 104. (After Sollas.)] The archaeocytes are rounded amoeboid cells early set apart in the larva; they are practically undifferentiated blastomeres. Some of them become reproductive elements, and thus afford a good instance of "continuity of germ plasm," others probably perform excretory functions.[198]