ly comes back to its normal position when the
pressure is relieved. If we examine the body of a sponge we shall notice
that there are occasional large openings into it, but everywhere
surrounded by smaller ones. If we should capture a live sponge and place
it in an aquarium with sea water, where we could study it, we should
find a circulation constantly going on, and that water was constantly
sucked in at the smaller openings all over the outside of the sponge and
as continuously ejected from the large openings. This process
constitutes what corresponds in the higher order of animals to both
respiration and blood circulation, combined. The sponge feeds upon
substances that are gathered up from the sea water, and breathes the air
contained in the same, so that it breathes, eats, and drinks through the
same set of organs.

When we first capture a live sponge from the sea it has a slimy, dirty
appearance, and is very heavy. The sponge is found to be filled with a
glutinous substance that is the fleshy part of the animal. It is very
soft and jelly-like, and after the sponge is dead it is readily squeezed
out, by a process which is called "taking the milk out," which leaves
simply the skeleton, the only useful part as an article of commerce.
This fleshy substance, in life, has somewhat the appearance and
composition of the white of an egg.

The mechanical process by which the sponge takes its nourishment is
exceedingly interesting. There are small globe-shaped cells with
openings through them that are lined with little hairlike projections
that move in such a manner as to suck the water in at one side of the
cell and push it out at the other. These little fibers are technically
called "cilia." We might describe them as little suction pumps that are
located at many points in the sponge, all acting conjointly to produce a
circulation through the finer openings or capillary vessels and finally
discharging into the larger chambers which carry off the residue. If we
should analyze the water as it is sucked into the sponge and that which
issues from it through the larger openings, we should find a difference
between the two. The expelled water would contain more or less carbon
dioxide.

There are many different varieties of sponge, and, while they all
possess certain characteristics in common, they are still very different
in many respects. Some of them are large and coarse, while others are
exceedingly soft and velvety. What i