Sponges vary greatly in shape, size, color, surface, rigidity, canal systems, and skeleton. They are cake-shaped, tubular, digitate, palmate, cup-shaped, vase-shaped, cone-shaped, spherical, hemispherical, pedunculate, etc., their shapes depending upon whether their growth is uniform or is excessive in a horizontal or in a vertical direction. When they grow evenly in both directions massive uniform shapes arise. If lateral growth predominates, broad, low, and incrusting shapes result. When there is an excess of vertical growth the forms are digitate; [pg104] but if vertical growth is not greatly in excess and is restricted to the marginal part, cup-shaped forms result, and tubular forms when lateral growth is particularly restricted. In some varieties tubular masses coalesce; sometimes flat forms unite in intricate compressed folds. Differences in rapidity of vertical growth make undulations more or less marked, producing lobes and protuberances. Sometimes narrow vertical growth is retarded and horizontal growth predominates, forming various shapes on apparent stems. Many of the horny sponges are colored, in shades of yellow, brown, red, and violet; some are black. Sponges are divided into two classes, the =Calcarea= and =Non-Calcarea=. The former have calcareous skeletons, which make them hard and rigid; the latter have skeletons of spongin-fiber or of silicious spicules, or of the two combined. All sponges, with the exception of one family (the /Spongillidæ/, which live in fresh water), are inhabitants of the sea, and live at various depths. The sponges of commerce belong to the /Non-Calcarea/ and are all confined to the genus /Spongia/. In this genus the skeleton is more or less flexible, being composed of spongin. They are shallow-water species, are confined to seas where the waters are of comparatively uniform high temperature, and flourish best when protected by reefs and islands. There are fisheries for sponges in the waters of the West Indies, the Bahamas, on the southern and western coasts of Florida, and in the Mediterranean and Red seas. Those of the Mediterranean surpass in quality the sponges of our coasts.