The neat round holes on the upper surface of a bath-sponge, into which a man's smallest finger will just go, have a very defi nite size for a good mechanical reason. They are known to zoolo gists as the "vents" or "oscula," and in healthy life water issues from them continuously so long as they are open. The large tube or cavity from which a vent opens (fig. 8) is the "cloaca," or main drain. The water issues from the cloacae through numerous round holes (fig. 8), and water enters to replace it by minute holes all over the surface, in most sponges invisible except with a lens ; the name of "pores" is technically confined to them, giving sponges their scientific name of Porifera. Except for its million pores, a sponge has no mouth.
From the pores the water enters a system of spaces and tubes branching repeatedly, and from them arise millions of delicate capillaries only some 8,u of an inch) in diameter. Each of these opens into a tiny cup-shaped chamber (fig. 6) known as a "flagellate chamber," because its walls are composed of ioo or 200 of the flagellate collar-cells about 6,u high. These wave their flagella, i o,u long, like so many eels fixed with their gills in a net and their tails downstream, and suck the water in through the fine hole or holes in the dome, driving it out into another fine tube, an efferent canal, which leads the water away. The efferent canals unite together into tubes which unite again into larger tubes, and these into a few main canals which open into the cloaca, and so to the vent (fig. 8). In the cleaned skeleton the canals cannot be traced to their smaller branches, but if a living sponge be killed in strong spirit its whole canal-system can be traced out.
Certainly in the Glass Sponges, and probably in some other sponges, cells in the cavities under the skin and elsewhere also feed, but it has been proved by experiment that the collar-cells feed, and in calcareous sponges only the collar-cells feed. There is a good reason for the flagellate chambers being the region in which food is taken in : because from their enormous number the aggregate size of the channel through which the water is pass ing is largest there, and consequently its motion is slowest, and there is least difficulty in arresting the food. Similarly in a sewage farm the water enters and leaves the field by a quick stream, but moves through the many little ditches so slowly that the land can absorb the manure.
Each flagellum with each stroke draws and pushes a little water forward into the chamber, and as the chamber was already full, its walls are stretched and the water inside it is under pressure, which is communicated, with some loss from friction, through the slowly moving water of the efferent canals to that of the cloaca.
The water inside the sponge is nowhere moving at so high a speed as in the current escaping from the vent, but the flagellate cham bers, efferent canals and cloaca form a pressure-chamber, from which a jet escapes through the vent. The pressure in the sponge's cloaca is kept up by the continuous work of the flagella, the f orce pumps of the sponge's hydraulic engine, sucking the water in through the pores and delivering it in a continuous jet from the vent.
We know that, when watering a garden with a hose-pipe without any nozzle a wide jet of water will fall to the ground only three feet away, but that diminishing the opening with the finger will result in a ten- or a twenty-foot jet. A smaller volume escapes, but therefore the water inside the hose moves more slowly, there is less loss of head by friction inside the hose, and the jet issues at a higher velocity. We thus understand why the sponge's vent is so small in comparison with the total width of channel of its many millions of flagellate chambers. If the vent were double the size, the current in all the canals and chambers would be speeded up, and the head maintained by the flagella would conse quently be lost in friction : the jet would be larger but much slower and would not travel so far.
It is most important for a sponge in still water that its vent jet should go far. The sponge lives by filtering its food from the water, and filters it completely, so that it can get no nourishment by taking in the same water again. It lives, also, by taking in oxygen and throwing out carbonic acid and the nitrogenous waste products of its food : so that the water which it has used is not only deprived of food, but poor in life-giving oxygen and loaded with deleterious carbonic acid and excreta. This valueless and poisonous water must be thrown far away from the entering stream.
Sponges are organisms which live on minute particles filtered from the water, and are shaped so as to separate the used water from their new supply. The history of sponges is the history of their adaptation to separate efficiently used outgoing water from the incoming water on which they feed.