8 INTRODUCTION 
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 pro- 
fusion, 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 plentifu'- 
ness. 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 sup- 
port. 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. Calecareous 
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 iso- 
pods, 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 
