A Monograph of the Existing Crinoids (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1915 Excerpt: ... unproven among the echinoderms. A crinoid attached by the adherence of the central plate to some solid object upon the sea floor would be subjected to a certain amount of strain from wave motion, or from the unequal movements of its own arms, as well as from the passage of other organisms. This strain would be felt along the suture connecting tliis central plate with the basals (or the infrabasals), and along the interbasal sutures. There are two ways of meeting this condition: (1) the basals (or the infrabasals or both) may become more upright and more nearly parallel with each other and fuse solidly with the central plate (now become a thick stalk); this has occurred in Holopus; (2) a second central plate, exactly similar to the original one, may be formed within--that is, ventral to--it, leaving a ligamentous articulation between them by which the strain is taken up, and this process may be continued indefinitely until a long articulated stalk is formed. A column formed by this process would of necessity be composed of very numerous and very short columnals, for the columnals would be attached to each other not by true articulations but by loose sutures; the amount of possible accommodation at a loose suture is far less than that at a true articulation, in which an articular fulcral ridge is developed and the ligament fibers have become segregated into two bundles one on either side of it, and therefore many such loose sutures must be developed in a given length of column to do the work of a single articulation. This explanation derives the crinoid stem from the original central plate equally well with the first, while at the same time it indicates the formation of the columnals from their first inception by a continuous twinning or reduplicative proces...