_Body wall_, the outer wall in bodies of the many-celled animals.
_Central cavity_, the cavity surrounded by the body wall in the simpler many-celled animals, as in the sponges.
_Canals_, channels through the body walls of sponges.
_Inhalent pores_, the outer ends of the canals.
_Osculum_, the large opening of the central cavity, at the distal end of the sponge.
_Spicules_, tiny needles of mineral substance found in the walls of many sponges.
_Fibers_, flexible threads of horny material found in the walls of many sponges.
_Endoderm cells_, cells lining the canals. They have cilia or flagella (projections larger than cilia).
_Ectoderm cells_, cells covering the outside of sponges and some other animals. In sponges it is believed that endoderm and ectoderm cells are able to exchange positions and functions.
_Mesoglea_, a jelly-like layer between the endoderm and ectoderm layers. In the sponges this contains many wandering cells, probably from the other layers.
_Porifera_ (pore bearers), animals with many more or less independent cells, supported by solid skeletal parts and penetrated by a system of canals which open on the surface as pores.
Study a simple sponge to see the shape, size, and point of attachment. Identify the osculum. In a diagram of a long section of a simple sponge identify the central cavity, body walls, canals, inhalent pores, ostia, and osculum. In a simple sponge cut like the diagram identify the same structures. Do the same for the toilet sponge.
Study a diagram of a portion of the body wall, considerably enlarged. Identify the endoderm and ectoderm cells, the spicules or fibers, and, among the spicules or fibers, irregular amœboid cells, sometimes called mesoderm cells.
Examine a fragment or section of each kind of sponge under the microscope. Notice the arrangement, shape, and length of the spicules and of the fibers.
Test both kinds of sponges by dropping a bit of each into weak acid, and noting the results. Also burn a bit of each and notice the odor.
1. What is the shape of a simple sponge? What enables a mass of cells to retain such a definite shape?
2. What seems to be the composition of the skeletons? Why is one type of skeleton rigid and the other elastic?