The second group of sponges is referred to
as glass sponges or Hyalospongiae. They have a skeleton composed of
six-rayed spicules of silica. The spicules are sometimes set
together into a network. Some skeletons are so pretty to look at
that collectors treasure them.
The third group is the Calcispongiae, or
calcareous sponges. These are marine sponges that have needle like spicules of lime.
A distinct feature of sponges is a porous skeleton of
interlocking spicules (bony, needle-like structures), glasslike
rods, or fibers. A thin, slimy, usually dark epidermal layer
consisting of flat cells called pinacocytes covers the body of a
living sponge. The perforations in the body lead through the skin to
a central cavity called the spongocoel, which is lined with
flagellum-bearing cells. These structures create currents that draw
water into the spongocoel through the perforations. Between the
outer and inner cell layers there lies a jellylike substance, which
contains free-moving cells known as amoebocytes.
Just as they come in different sizes and shapes, their living
patterns are different too. They may live in groups or colonies or
they may choose to remain as solitary animals attached to the sea
bottom or to other solid objects. Some sponges live in close
association with other animals. A certain species of crab, for
instance, is always covered with a growth of sponge, which in turn
serves as a camouflage for the crab. Another variety of crab
transplants sponges into its body.
One of the most commonly used species of commercial sponge is
Spongia officinalis, sometimes called the glove sponge. Sponges
are caught by hooks or harpoons. Sometimes they are cut off the
bottom of the sea by divers. The so-called bath sponge, or
commercial sponge, is obtained mainly from the eastern Mediterranean
and from the Gulf of Mexico and the Caribbean Sea (off the western
coast of Florida and the West Indies and elsewhere), where they are
found at depths of less than two hundred feet.