As already learned in the examination of examples of one-celled animals, it is evident that life may be successfully maintained without a complex body composed of many organs performing their functions in a specialized way. The marine Protozoa illustrate this fact admirably. Despite their lack of special organs and their primitive way of performing the life-processes, that they live successfully is shown by their existence in such extraordinary numbers. They outnumber all other animals. The conditions of life in the surface-waters of the ocean are easy and constant, and a simple structure and simple method of performing the necessary life-processes are wholly adequate for successful life under these conditions. FOOTNOTE: [7] In some Protozoa a number of similar cells temporarily unite to form a colony, but each cell may still be regarded as an individual animal. CHAPTER XVI BRANCH PORIFERA: THE SPONGES THE FRESH-WATER SPONGE (_Spongilla_ sp.) TECHNICAL NOTE.--Fresh-water sponges may perhaps not be readily found in the neighborhood of the school, but they occur over most of the United States, and careful searching will usually result in the finding of specimens. They are compact, solid-looking masses, sometimes lobed, resting on and attached to rocks, logs, timbers, etc., in clear water in creeks, ponds, or bayous. They are creamy, yellowish-brown or even greenish in color and resemble some cushion-like plant far more than any of the familiar animal forms. They can be distinguished from plants, however, by the fact that there are no leaves in the mass, nor long thread-like fibres such as compose the masses of pond algæ (pond scum). When touched with the fingers a gritty feeling is noticeable, due to the presence of many small stiff spicules. Sponges should be removed entire from the substance they are attached to, and may be taken alive to the laboratory. They die soon, however, and should be put into alcohol before decay begins. Note the form of the sponge mass. Is it lobed or branched? Examine the surface for openings. These are of two sizes; the larger are _osteoles_ or _exhalant openings_, while the smaller and more numerous are _pores_ or _inhalant openings_. The sponge-flesh is called _sarcode_. Examine a bit of sarcode under the microscope; note the _spicules_. Have these spicules a regular arrangement? Of what are they composed? Draw the entire sponge, showing shape and openings; draw some of the spicules.