Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/662

640 consists usually of five interradial, and five notched or per forated radial plates united by muscles the homologues of the auriculae of the Echinidea. A circlet of plates, in certain species, surrounds the anus. The fluid filling the various canals of the ambulacral system contains nucleated cells. The ambulacral circular vessel lies behind the calcareous ring ; it gives off from one to five or more Polian vesicles, also one or more madreporic canals. Calcareous spiculoe are contained in the wall of the madreporic canal, and often it is terminated at the free end by a cribriform plate similar to a madreporite. By means of the madreporic canal the ambulacral vessels communicate with the peritoneal cavity. From the ambulacral ring there proceed five, sometimes more tentacular canals, which supply ctecal branches to the ten tacles, and usually, also, five ambulacral canals, which give rise to as many rows of pedicels situated in most cases radially, and forming a dorsal bivium and ventral trivium. Sometimes, as in Psolits, the pedicels are irregularly dis persed over the whole perisome, or they may be met with only on the lower surface of the body, where they subserve locomotion. In Molpadia and its allies there are ambula cral canals, but no pedicels ; in the Synaptidce the canals are wanting. The sexual organs are one or two groups of branched tubuli, which open either on the dorsal surface or between the two dorsal tentacles. Except in the Synaptidae, and apparently also the LiodermatidoB, the sexes are dis tinct. The nervous system consists of a circular cord, lying above the ambulacral ring, and giving off five apparently hollow branches, which pass through holes or notches in the radial plates of the calcareous ring, to pro ceed down the centre of the longitudinal muscular bands of the body.

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{{ti|1em|{{11fine|Mr C. Moore, E.G.S. (J{&amp;lt;:p. lint. Aasoc., 1872, p. 117), has de scribed wheel-like spiculae of four species of Clnrodota, one from the Inferior Oolite, one from the Upper Lias, and two from the Middle Lias. &quot;They are formed of a number of minute wheel-spokes, varying from 5 to 13, which start from a central axis, and are sur rounded on the outside by a wheel-tire; on the inner edge of some species are a scries of very minute tocth, extending over the contra! cavity.&quot; Mr R. Etheridge, jun. (in the Memoirs of the Gcol. Survey of Scotland, Explanation of Sheet 23, 1873), has called ^attention to the discovery by Mr J. Bonnie, survey-collector, of similar organ isms in the Lower Carboniferous Limestone group of E. Ivilbride, and in shales of the Upper Limestone group of Williamwood, near Glasgow.}}}}

1em  ECHO, in Greek mythology, one of the Orcades or mountain nymphs. The word denotes mere sound ; and the stories told of her are so transparent that they can scarcely be said to belong to the class of fully-developed myths. As Selene to the Greek was clearly the moon, so Echo was the being who could not speak until she was spoken to. and then could only repent the last words of the speaker. This penalty is said to have been inflicted upon 