Let us examine the arithmetical facts which make it advan tageous for unicellular collar-flagellates to unite into a sponge and for elementary thimble-shaped sponges such as the Olynthus to unite into a second-grade sponge like Sycon (fig. 4) in the Chalky Sponges, or like the "Rhagon"; the name given to very young Four-ray Sponges. Assume that in fig. 8, which illustrates a young Lace-chalk, that the size of the opening of the vent has been drawn exactly equal to the aggregate size of the vents of one hundred thimble-shaped flagellate chambers represented as making up the sponge. There is therefore no advantage from increased speed through a narrow vent, the speed of the water as it leaves the whole sponge being that with which it leaves the component thimbles, but the figure shows the immense advan tage which has nevertheless been gained to each component or thimble by combination with its neighbour. Whereas the hundred independent thimbles would have a hundred independent streams, each with its own surface rubbing against the surrounding station ary water and so bringing the jet speedily to a standstill, it will be seen that of the component streams issuing from the com mon vent only 36 are external, and of these only half the surface is exposed to friction. The jet from the vent has the same velo city as the hundred streams, with their full aggregate weight, but is exposed to less than one-fifth of the total resistance to which the hundred separate streams were exposed. Experimentally the combined jet goes to about ten times the distance to which the individual jet travelled, and for each thimble the water which it sucks in will have, say, the pollution from used water which it would have had if isolated. The principle is akin to the reduction of overhead expenses when small businesses unite.
Consider the action of the currents around a live bath-sponge at the bottom of a still sea. When the jet from the sponge throws water from the bottom layer of the sea to the layer four feet above it, an equal quantity of water must be pushed out of the four-foot layer to go somewhere ; and an equal quantity must come into the bottom layer. If the water be still, these two require ments will be satisfied at once by the formation of a closed eddy like a smoke-ring, which will supply the water which is sucked into the pores by bringing down the water which has been ejected by the vent, so that the work of the flagella will be wasted and will bring to the sponge neither food nor oxygen.
Water in the sea is rarely absolutely still, even at the bottom on a windless day. Even in the Levant end of the Mediterranean there must be a slight tidal drag for half the day and a shore ward drift when the sun is hot and a seaward drift at night. Such currents, however slow, will bear the outgoing jet to one side : if the jet be only six inches high, the foul water will still come down to pollute three-quarters of the intake; if it be four feet high the whole return will come to the ground free of the sponge, and the purity of the intake will be absolute. The longer the jet from the vent, the purer will be the water of the intake. It has been shown
for certain sponges by calculation (7) that the diameter of the vent is that which will carry the water to the greatest possible distance for that make of sponge.
Yet in a fine Turkey cup-sponge, however, we find there is no narrow vent at all, but all the efferent cloacae open into a cup shaped false cloaca, which widens as it goes upwards to a lip sev eral inches across. This is a second method of those arrived at dur ing the 1,400 million years that sponges have existed; by it the angle between the outgoing and incoming streams is increased from the right angle of the bath-sponge, and becomes exceedingly wide; so that, however slow the outgoing stream, the diameter of the circular eddy is so large that the slightest drift of the surrounding water must take the polluted effluent away from the intake. In the fan sponge (fig. 3), where the vents and pores are on opposite sides of an upright plate, the angle between intake and effluent is 180°, so that the used water never returns. This, however, is only advantageous where the drift of the sea is un changing in direction. Such a condition exists at the bottom of the great oceans, where there is a slow current from pole to equator, and in some of the deeps around Scotland ; but in a tidal run the sponge for a third of its life would have the tide pressing the water into the vents and out of the pores. In the tidal English Channel the peculiar Cannon-sponge accomplishes the advan tageous result in a wholly different manner. It points its wide vent horizontally, looking like an 18th century cannon; but its round base is carried on a thin stalk like a pig's tail, so that as the tide sweeps round it, swivels with it, and always points down stream. A deep-sea Claw-sponge, known as the Umbrella-sponge has also a flexible stalk so that the convex side of the umbrella must be down stream.
Appreciating the advantages of completely separating outflow from intake, it is possible to conceive how sponges arose, and such very similar forms have been evolved along so many different lines of descent. Glass Sponges appear as the offspring of branch ing colonies of unicellular collar-flagellates which learned to secrete a substance crystallising into rectangular scaffoldings, on which they can set themselves like a salmon-net across the slow currents of the deep ocean and so obtain an end less supply of the suspended organic par ticles. They have existed since the earliest fossiliferous rocks, and probably twice as long; but with no hydraulic system to form a forcible jet from the vent, they have been unable to live except in oceanic currents. The True Sponges are the off spring of multicellular gelatinous colonies of collar-flagellates (such as Proterospon gia); they have learned to shape their gelatinous aggregate so as to unite the cur rents made by individual collar-cells, and on various plans to direct the streams of used water away from the approaching I drift of purer water; thus they can sup port existence in still caves and tide pools.