A stemmed Echinoderm of special interest, first known from the Ordovician, has persisted to the present day. It is the so-called "sea lily" or "stone lily," consisting of a complex, headlike portion attached to the sea bottom by a long segmented stem, the whole being supported by lime carbonate. They were very numerous during the Silurian, but they seem to have culminated in variety of species and numbers of individuals during the Mississippian period when they were exceedingly profuse. Hundreds of species of "stone lilies" are known from Mississippian strata alone, and in certain localities, as at Crawfordsville, Ind., and Burlington, Ia., the "stone lily" remains are so numerous that when living they must have literally forested parts of the sea bottom. From Mississippian time to middle Mesozoic time they occupied a relatively subordinate position when they again developed in great profusion. The Mesozoic forms were distinctly more like those of to-day, and it scarcely seems credible that any creature could have contained such a multiplicity of hard parts, more than 600,000 segments having been counted in a single fossil from Jurassic strata. The "sea lilies" of to-day are relatively unimportant. The familiar five-pointed "starfishes," so common along our seacoasts, are first known from the Ordovician, and they persisted through the many millions of years to the present time with remarkably little change. The so-called "sea urchins" live in rounded, segmented lime-carbonate shells bristling with movable spines. "Sea urchins" are first known from the Ordovician, but they did not become abundant and diversified until Mesozoic time, when many of them took on a very modern aspect. Worms are known to have existed ever since late Proterozoic time, as proved by the occurrence of tracks, borings and more rarely delicate impressions on rock surfaces. Because of their softness they have rarely been well fossilized and are, therefore, of no great evolutionary or geological importance. [Illustration: Fig. 52.--Fossil brachiopods belonging to the subkingdom of animals known as "Molluscoids": a, b, c, forms characteristic of the Ordovician, Devonian, and Triassic periods, respectively.]