=Special Report=: _Sponge “Fisheries.”_ (Localities; how sponges are taken, cleaned, dried, shipped, and sold.) CHAPTER IV POLYPS (CUPLIKE ANIMALS) THE HYDRA, OR FRESH-WATER POLYP [Illustration: FIG. 33.—A HYDRA. ] SUGGESTIONS.—Except in the drier regions of North America the hydra can usually be found by careful search in fresh-water ponds not too stagnant. It is found attached to stones, sticks, or leaves, and has a slender, cylindrical body from a quarter to half an inch long, varying in thickness from that of a fine needle to that of a common pin. The green hydra and the brown hydra, both very small, are common species, though hydras are often white or colourless. They should be kept in a large glass dish filled with water. They may be distinguished by the naked eye but are not studied satisfactorily without a magnifying glass or microscope. Place a living specimen attached to a bit of wood in a watch crystal filled with water, or on a hollowed slip, or on a slip with a bit of weed to support the cover glass, and examine with hand lens or lowest power of microscope. Prepared microscopical sections, both transverse and longitudinal, may be bought of dealers in microscopic supplies. One is shown in Fig. 39. [Illustration: FIG. 34.—Forms assumed by Hydra. ] Is the hydra’s =body= round or two-sided? (Fig. 35.) What is its _general shape_? Does one individual keep the same shape? (Fig. 34.) How does the length of the threadlike _tentacles_ compare with the length of the hydra’s body? About how many tentacles are on a hydra’s body? Do all have the same number of tentacles? Are the tentacles knotty or smooth? (Fig. 35.) The hydra is usually extended and slender; sometimes it is contracted and rounded. In which of these conditions is the base (the foot) larger around than the rest of the body? (Fig. 34.) Smaller? How many _openings_ into the body are visible? Is there a depression or an eminence at the base of the tentacles? For what is the _opening_ on top of the body probably used? Why are the tentacles placed at the top of the hydra’s body? Does the _mouth_ have the most convenient location possible? [Illustration: FIG. 35.—HYDRA (much enlarged). ] The conical projection bearing the _mouth_ is called _hypostome_ (Fig. 34). The mouth opens into the _digestive cavity_. Is this the same as the general body cavity, or does the stomach have a wall distinct from the _body cavity_? How far down does the body cavity extend? Does it extend up into the tentacles? (Fig. 39.) If a _tentacle is touched_, what happens? Is the body ever bent? Which is more sensitive, the columnar body or the tentacles? In searching for hydras would you be more likely to find the tentacles extended or drawn in? Is the hypostome ever extended or drawn in? (Fig. 34.)