Fig. 2.—Circulation of Water through the Sponge.
In looking at any sponge you will notice large holes through it, with many small pores scattered between them. The living sponge is constantly drawing in water at the small pores. This water passes through a set of branching canals, and is thrown out from the large holes on the surface, as seen in Fig. 2. (The arrows show the direction of the current.) With a microscope little fountains may be seen constantly playing from the large holes of a living sponge. The circulation is kept up in the canals by the movement of "cilia," which are delicate threads waving gently but continually. The word cilia means "eyelashes"; let us remember it, for this is a name we shall often want to use. The cilia are shown in those cup-like hollow places in the canals (Fig. 2). The stream of water thus passing through the sponge brings to every part of it small particles of food, and all the air it needs for breathing purposes.
Everything that lives must eat and breathe, but how is the sponge to eat without a mouth? When the food touches any part of its body, the soft, jelly-like flesh sinks in to form a little bag; at the same time the surrounding parts creep out over the morsel of food, until it is entirely covered and digested. After this the flesh returns to its original position, and any shell or other refuse that remains from the meal is washed away.
Sponges have a curious manner of producing their young. At certain seasons very small oval masses of jelly are formed on the inner surface of the canals, which finally drop off. They remain in the canals for a time, and become perfect eggs, after which they are thrown out by the stream issuing from the fountains, and instead of falling to the bottom, as we might suppose such helpless masses of jelly would do, they swim around as if they meant to have a little sport before commencing the sober realities of life.
You will be interested to know that while these jelly-like eggs were resting in the canals of the parent sponge, delicate cilia (which we learned about just now) were forming at one end of the egg. These cilia strike the water with a rapid motion, and the eggs are rowed about through it until they settle down and attach themselves to some rock or shell on the bottom of the ocean, and finally grow up into the perfect sponge. The waters are swarming with these eggs at certain seasons, and great quantities of them are eaten by larger animals.
SPONGE-FISHING.
Sponges are common in nearly all parts of the world, and they differ greatly in size and quality, but few species being useful to man. Some species are nearly round, others are always cup-shaped, some top-shaped, and some branched. A fresh-water sponge is frequently found in our streams, growing upon sticks and stones. It is of a bright green, and when seen under the water in a flood of sunlight it is very pretty.
The spicules of sponges grow in a variety of elegant shapes, but they are visible only with a microscope. They are composed of lime or flint, and are generally sharp-pointed. They are imbedded in the flesh as well as in the horny fibres, thus serving to protect the helpless creatures from being devoured by fish and other animals. In our fine sponges, the skeleton is almost destitute of spicules, while in some others the flesh is supported wholly by spicules, giving them so loose a texture that they are of no value for domestic purposes.