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.
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 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]]
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]]
Fig. 67.—Free-swimming larva of Gellius varius, in optical section. a, Outer epithelium; pi, pigment; x, hinder pole. (After Maas.)
The reproductive elements are ova and spermatozoa, and are to be found in all stages in the dermal jelly. Dendy states that the eggs are fertilised in the inhalant canals, to which position they migrate by amoeboid movements, and there become suspended by a peduncle.