Order VI. Echinoidea, sea-urchins, sea-eggs, heart-urchins. SeXes distinct. See Order VII flolothuroidea, vermiform or slug-like echinoderms, with a leathery skin, in which calcareous granules and spicules are developed; mouth surrounded by a circlet of tentacles; larva vermiform, and without a skeleton; sexes usually distinct. The• members of this order are commonly known as trepangs, sea-cucumbers, etc., and are the most highly organized of all the echinodermata. There is a long, convoluted intes tine, and a special respiratory or water-vascular system is often developed in the form of arborescent tubes. At a certain period the young are barrel-shaped, having trans verse rings of cilia, by means of which they rotate rapidly on their long axis, and have been at this stage of existence described as a distinct genus, under the name of aurku laria. In the adult typical holothurians, locomotion is produced by means of rows of ambulacral tube-feet, or by alternate extension and of the body, but in some members the animal moves by means of spicula distributed in the integument. See HOLOTTIURIA.
The echinodermata began their existence in the lower Silurian formation, and their remains are found in most sedimentary rocks up to the present time. The cystoidea and blastoidea are extinct, and not more recent than paleozoic. Many crinoids are extinct, having their greatest development in Paleozoic time. In the t•iassic formation is found the beautiful stone-lily. In the Jurassic occurs the pear-encriuite, and in the chalk the tortoise-encrinite. Fossils of asteroidea abound in both upper and lower Silurian rocks, as paleogaster, a beautiful form (q.v.). Many rare and beautiful fossils abound in the oolite. as goniaster, phunaster, and uriaster. The ophiuroldea are rare fossils, the protaster Sedgwickii being an example, found in the Silurian; hut most of the members are more recent, many reaching to the present time. The echinoidea are represented in the paleozoic rocks by only one family, but numerous fossils are found in mesozoic and recent periods. The echinoids of the secondary and tertiary formations resemble present forms in not than twenty rows of calcareous plates.