^C. leptostyla.^ This species is found growing on /Fucus/, on the under side of stones at low-water mark, and in tide-pools, where it often covers several feet of the surface of the rock with a delicate velvet-like carpet. It is red in color and is, apparently, a soft and tender species, but it thrives on the most exposed beaches. The colonies are cylindrical tubes about one quarter of an inch in height, rising from a creeping stem (/hydrorhiza/). Each tube is surmounted by a zoöid with fifteen to thirty tentacles, which is constantly changing form by its contractions. Below the tentacles are reproductive buds arranged in clusters. Common from Long Island Sound northward. (Plate XLI.) [Illustration: PLATE XLI. Clava leptostyla. Tubularia indivisa. Hybocodon prolifer. Coryne mirabilis. Eudendrium ramosum. Tubularia Couthouyi.] GENUS ^Hydractinia^ ^H. polyclina.^ The soft, pinkish covering often seen on shells inhabited by hermit-crabs. This association of two different kinds of animals is known as /commensalism/, and is a partnership formed for the benefit of one or both the individuals. In this case the mossy appearance of the hydroid conceals the shell, while the stinging-cells with which it is invested are weapons of defense against the enemies of the crab and also help to paralyze its prey. In return for these favors the colony is moved about, thereby obtaining perhaps better oxygenation. Originally it was thought that /Hydractinia/ lived only on the shells occupied by hermit-crabs, and that the nomadic life was essential to its existence; but this is not the case, for it is also found growing on rocks in tide-pools. These colonies arise from a creeping stem, which forms a horny, root-like [pg123] network over a surface and develops at intervals projecting points on which the zoöids live. Each colony consists of feeding members, of reproductive members, and of a third kind which seems to have a protective function. These last are more slender than the others, and are without tentacles, but are armed with lasso- or stinging-cells. The colonies are of different sexes, the male being lighter in color than the female colonies. The eggs develop into planulæ, which swim about for a while and then give rise to other colonies. It is found from New Jersey northward, and is very abundant in Long Island Sound. GENUS ^Coryne^ ^C. mirabilis.^ A hydroid about one inch high, growing in patches and appearing like tufts of moss on rocks between tide-marks. When highly magnified it shows club-shaped tubes with pedicels, terminating in zoöids, scattered over the swollen ends. The medusa-bud is larger than the others and is lower on the tube. It liberates a swimming-bell, which is called /Sarsia/. (Plate XLI.) [Illustration: Colony of /Coryne/, natural size.] ^Sarsia^