Parker found that the pressure in the chambers of Stylotella (a Claw- s sponge) supports a column of water 34 to c 4 mm. above the surrounding surface (6). The velocity of the jet from the vent in Leucandra aspera is about 8 cm. a second, throwing 5 gallons a day to a distance of nine inches. The velocity at the same moment past the collar-cells (fig. 6) was 2 to 4 cm. an hour (7) taking I or 2 seconds to pass the cell 12µ high.
Chalky Sponges have no spongin, though the red and yellow pigments may be of an allied substance; Four-ray Sponges have little or none. Needle-sponges have commonly a basal spongin-plate, and their spicules are cemented by more or less spongin into bundles and networks, the claw-sponges showing so much spongin that some (as Ectyon and Ophlitospongia) may easily be mistaken for Horny Sponges ; among the Chalinine Spindle-sponges, the spicules are sometimes reduced to a few spindles or rods inside horny fibres. In Horny Sponges the spongin of the fibres is secreted by flask-shaped cells which are known as spongoblasts. An examination of the unmarketable sponge called Hircinia, shows that similar cells are forming the horny cuticle of the outer surface, and that the horny fibre is secreted by a tube of spongoblasts, pouched in from the sticky surface and carrying with it the adherent sand. (Projection of the fibre is not the condition in healthy life, but is due to the shrinkage as water drains from the sponge.) Spicules.—Sponge-spicules are formed by cells. In the calcite three-ray spicules the Chalky Sponges (Calcarea) each ray is formed by two cells, the first rudiment being apparently non crystalline, with the three rays separate (Minchin, 1898) ; then the rays become crystalline and unite, the three-ray spicule being optically one crystal (Sollas, 1885). The optic axis of the crystal is perpendicular to the plane of the rays in the symmetrical spicules of the Lace-chalks, and inclined to it in the upsilon shaped "alate" spicules (fig. 12) of the Banana-chalks. The spindles and club spicules of Banana-chalks are each formed by two cells (Bidder, 1898, Woodland, 19o5, Minchin, 1908). In Grapnel-sponges Dendy (Quart., Micr. Soc. 1926) found a slender non-siliceous rudiment, the "proto-rhabd," either laid dcwn by cells of unusual appearance called "formative cells," or, as he afterwards supposed, reproduced like a bacterium from a parent proto-rhabd; several sponge-cells (silico-blasts) gather round this rudiment and pour opal over it and the formative nuclei. In Glass Sponges Ijimi describes a large number of cells
as taking part in the formation of each spicule : the massed cells so engaged are the only cells of Hexactinellids, except the gem mule-cells, which show a gelatinous agglomeration such as is common in the True sponges.
The "collar" is not homogeneous, but a tubu lar tassel of sticky parallel rods—in Grantia 2o-30, in Clathrina or Halichondria fewer. The rods can contract, thickening as they do so (Julian Huxley, 1911) ; they are not proved prehensile. The cell contains food-reserve granules, transparent after starvation, and (Gatenby—see "Cytology") a Golgi body. In Chimney chalks the nucleus is near the top of the cell and connected with the flagellum (8, p. 21 Bidder 1898), in Lace-chalks it is con nected in young cells (Robertson & Minchin, 191o), but separates and passes to the base.
In many sponges especial fusiform cells form muscular bands and sphincters (fig. o) whose contraction either closes a channel for water or alters its width. No sponge has any nervous mechanism. The flagellate cells wave their flagella with out correlation, in different periods and at different phases. The contractile cells contract to direct stimulus, or are restrained from contraction by direct stimulus (b). There are no sensory cells and no nerve fibres.
Within the jelly of the sponge-body, ova and other cells wander about with amoeboid movements, sucking from the bases of the collar-cells their food-resources and eating any intrusive foreign particle or organism encountered in the jelly. In the fresh-water Spongilla (9) the collar-cells are described as throwing carmine into the jelly and the amoeboid cells as devouring it; the green unicellular alga which colours Spongilla grows uninjured while Spongilla is well fed, but forms a reserve, being eaten by the amoeboid cells when other supplies fail.