_To preserve Spongillidæ._--Spongillidæ must be preserved dry or in very strong alcohol. Formalin should not be used. _To clean siliceous sponge spicules._--Place small fragments of the dried sponge (if alcohol is present, the reaction is apt to be violent) in a test tube, cover them with strong nitric acid and boil over the flame of a Bunsen burner or small spirit lamp until the solid particles disappear. Add a large quantity of water to the acid and filter through pure cellulose filter-paper, agitating the liquid repeatedly. Pass clean water in considerable quantities through the filter-paper and dry the latter carefully; place it in a spirally coiled wire and ignite with a match, holding the wire in such a way that the spicules released by the burning of the paper fall into a suitable receptacle. They may then be picked up with a camel's-hair brush and mounted in Canada balsam. _To examine the skeleton of a Spongillid._--Cut thin hand-sections with a sharp scalpel, dehydrate if necessary, and mount in Canada balsam. _To prepare gemmules for examination._--Place the gemmules dry in a watch-glass with a few drops of strong nitric acid. When gas is given off freely add water in considerable quantities. Remove the gemmules with a camel's-hair brush to clean water, then to 50%, 70%, 90% and absolute alcohol in succession, leaving them for an hour in each strength of spirit. Clear with oil of cloves and mount in Canada balsam. _To ascertain the presence of bubble-cells in the parenchyma of a Spongillid._--Tease up a small piece of the sponge with a pair of needles, mount under a thin cover-slip in strong spirit, and examine under a high power of the microscope. _To preserve Hydra in an expanded condition._--Place the polyp in a watch-glass of clean water and wait until its tentacles are expanded. Heat a few drops of commercial formaldehyde and squirt the liquid while still hot at the _Hydra_, which will be killed instantaneously. Remove it to a solution of formaldehyde and spirit of the following formula:-- Commercial formaldehyde 1 part. Absolute alcohol 3 parts. Distilled water 7 parts. Then pass the _Hydra_ through 50% and 70% alcohol and keep in 90%. _To examine the capsules of the nettle-cells._--Place a living _Hydra_ in a small drop of water on a slide and press a thin cover-slip down upon it.