[Illustration: FIG. 75.—_Proterospongia haeckeli._ _a_, Amoeboid cell; _b_, a cell dividing; _c_, cell with small collar; _z_, jelly. × 800. (After S. Kent.)] {183}CHAPTER VIII PORIFERA (_CONTINUED_): FORMS OF SPICULES—CALCAREA—HOMOCOELA—HETEROCOELA —HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA—MONAXONIDA—CERATOSA—KEY TO BRITISH GENERA OF SPONGES Sponges fall naturally into two branches differing in the size of their choanocytes: in the MEGAMASTICTORA these cells are relatively large, varying from 5µ to 9µ in diameter; in MICROMASTICTORA they are about 3µ in diameter.[212] For further subdivision of the group the SPICULES are such important weapons in the hands of the systematist that it is convenient to name them according to a common scheme. This has been arrived at by considering first the number of axes along which the main branches of the spicules are distributed, and secondly whether growth has occurred in each of these axes in one or both directions from a point of origin.[213] I. _Monaxons._—Spicules of rod-like form, in which growth is directed from a single origin in one or both directions along a single axis. The axis of any spicule is not necessarily straight, it may be curved or undulating. The ray or rays are known as actines. Biradiate monaxon spicules are termed "rhabdi" (Fig. 76, _a_). A rhabdus pointed at both ends is an "oxea," rounded at both ends a "strongyle," knobbed at both ends a "tylote." By branching a rhabdus may become a "triaene" (Fig. 110, _k_, _l_). Uniradiate monaxon spicules are termed "styli." II. _Tetraxons._—Spicules in which growth proceeds from an {184}origin in one direction only, along four axes arranged as normals to the faces of a regular tetrahedron. Forms produced by growth from an origin in one direction along three axes lying in one plane are classed with tetraxons. III. _Triaxons._—Spicules in which growth is directed from an origin in both directions along three rectangular axes. One or more actines or one or two axes may be suppressed. IV. _Polyaxons._—Spicules in which radiate growth from a centre proceeds in several directions. V. _Spheres._—Spicules in which growth is concentric about the origin. A distinction more fundamental than that of form is afforded by the chemical composition: all sponges having spicules composed of calcium carbonate belong to a single class, CALCAREA, which stands alone in the branch Megamastictora. [Illustration: FIG. 76.—Types of megascleres. _a_, Rhabdus (monaxon diactine); _b_, stylus (monaxon monactine); _c_, triod (tetraxon triactine); _d_, calthrop (tetraxon tetractine); _e_, triaxon hexactine; _f_, euaster.] BRANCH I. MEGAMASTICTORA CLASS CALCAREA