17. Name other diseases caused by protozoa. How are they fought? 18. What beneficial effect have some protozoa upon the water of stagnant ponds and ditches? How may some forms injure water for household purposes? 19. Give at least three reasons for thinking that protozoa are the most ancient animals. 20. Why are protozoa of great importance to the world? 2. A STUDY OF SPONGES _To show how cells loosely associated may work together_. _Materials._ The simplest of the many-celled animals are the sponges, which, with one exception, are salt-water forms. That one, the spongilla, is not easily found and is very difficult to maintain in the laboratory. For these reasons the material for this study is very meager, except at the seashore, and much of the work must be done from diagrams and reference books. Small simple preserved sponges and complex toilet sponge skeletons will also be used. _Definitions._ _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. _Ostia_, the inner 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. _Directions._ 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.