The sertularians are distinguished by the horny cup, which is sessile—that is, set directly against the stem instead of being raised upon a stalk. They are among the most common objects of the beach, and, like the plumularians, are often mistaken for plants by the amateur collector and are gathered and pressed as seaweeds. They are found everywhere along the coast. They zigzag over the fronds of seaweeds or hang in fringes upon them, as well as upon rocks, stones, and shells. They well repay close examination with a glass. Every open cup bears a wreath of tentacles, which makes the branch a spray of stars. This is not an inappropriate comparison, for besides their starry shape some species emit a phosphorescent light. [Illustration: PLATE XLII, B. Sertularia argentea. Sertularia cupressina.] [Illustration: PLATE XLIII. Plumularian hydroid. Aglaophemia struthioides. Aglaophemia struthioides, magnified. Plumularia falcata.] GENUS ^Sertularia^ ^S. pumila.^ The most abundant of all the hydroids on the northeast coast is this species, which is found in profusion upon /Fucus/ and other seaweeds, and mingled with them upon the rocks. It is easily distinguished from the campanularians because its zoöid-cups are close against the stems (sessile) instead of on stalks or pedicels. The stem creeps [pg127] over the fronds of seaweeds, often crossing and recrossing in a tangled mass. At short intervals the upright, straight branches rise to one inch or one and a half inches in height, and are more or less branched. All except the creeping stems are close set on each side, with cylindrical zoöid-cups which turn outward at the ends. The cups of the reproductive zoöids are not sessile; they are much larger than those of the nutritive ones and are urn-shaped. (Plate XLII, A.) ^S. argentea.^ This is a beautiful species, common from New Jersey northward. It has a profusion of silvery branches on a dark stem. The colonies are often a foot or more long, and the branches at the top and bottom of the stem are shorter and fewer than those in the middle of the colony. The zoöid-cups are nearly cylindrical, pressed closely to the stem, nearly opposite or subalternate to one another, and end in pointed tips. The medusa-bud is urn-shaped, with two horns at the top. (Plate XLII, B.) ^S. cupressina^, the sea-cypress. This species is similar to /S. argentea/, but the main stem is thicker and longer, and the branches less crowded and less subdivided. The branches are arched or drooping, instead of straight, and gradually decrease in length at some distance from the lower and upper parts of the stem, giving a spire-like apex, the stem often continuing into a bare, branchless extremity. The zoöid-cups are tubular, not much narrowed or divergent above, and two-lipped on the margin. It is found from New Jersey northward. (Plate XLII, B.) THE PLUMULARIANS