Rocky shores furnish an abundance and great variety of objects to the collector. The seaweeds here find places of attachment, and the lee and crevices of the rocks afford shelter to many animals which could not live in more open and exposed places. The [pg008] rock pools harbor species whose habitat is below low-water mark and which could not otherwise bear the alternation of the tides. The first objects on the rocky beach to attract attention are the barnacles and rockweeds. They are conspicuous in their profusion, the former incrusting the rocks with their white shells, and the latter forming large beds of vegetation; yet both are likely to be passed by with indifference because of their plentifulness. They are, however, not only interesting in themselves, but associated with them are many organisms which are easily overlooked. The littoral zone is so crowded with life that there is a constant struggle for existence,—even for standing-room, it may be said,—and no class of animals has undisputed possession of any place. Therefore the collector should carefully search any object he gathers for other organisms which may be upon it, under it, or even in it, such as parasites, commensals, and the organisms which hide under it or attach themselves to it for support. Let the rockweed (/Fucus/) be carefully examined. Among the things likely to be found attached to its fronds are periwinkles (/Littorina litorea/), which simulate the plant in color, some shells being striped for closer mimicry. Sertularian hydroids also are there, zigzagging over the fronds or forming tufts of delicate horny branches upon them. Small jelly-like masses at the broad divisions of the fronds may be compound ascidians. Calcareous spots here and there may be polyzoans of exquisite form, while spread in incrusting sheets over considerable spaces are other species of /Polyzoa/. Tiny flat shelly spirals are the worm-cases of /Spirorbis/. A pocket-lens is essential to enable one to appreciate the beauty of these minute forms. Under the rockweeds are many kinds of crustaceans; perhaps there will also be patches of the pink urn-like egg-capsules of /Purpura/ at the base of the fucus. Various kinds of seaweed abound in the more sheltered parts of the rocks, and among them will be found amphipods and isopods, many of which are of species different from those of the sandy beaches. Here, too, is the little /Caprella/, imitating the seaweed in form, and swaying its lengthened body, which is attached to the plant only by its hind legs. On the seaweeds, as well as in the tide-pool, may be found beautiful hydroids, and on [pg009] them the curious little sea-spiders (/Pycnogonidæ/), animals which seem to be all legs.