Bulletin - United States National Museum Volume 1 (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 edition. Excerpt: ... These should be regarded as primary plates, though not always occurring in the young; if present at all they appear and disappear again in a very short space of time. (2) The secondary or perisomic skeleton.--This consists of the side and covering plates, the plates of the disk (excepting the orals), and of the brachial perisome, and the numerous minute plates and spicules mostly lying toward the inner side of the soft integument, ordinarily more or less isolated, but sometimes slightly connected by strands of connective tissue. The perisomic plates of the so-called secondary series differ from the primary plates, among other ways, in possessing great variability, or exhibiting an absence of fixity, in their shape and in the method and manner of their occurrence; in other words, they are directly dependent upon local mechanical conditions, while the phylogenetically significant primary plates, originally just as dependent upon local mechanical conditions, have, through long existence as integral units, attained a distinct entity of their own, which is to a certain degree dominant over the mechanics of theu: immediate surroundings. Among the recent crinoids the interradials (and the radianal) are, through degeneration, somewhat intermediate in character between this series and the one preceding; the well-developed plates on the disks of the young of the various comasterids and of Thaumatocrinus which are resorbed before the adult condition is attained, also show in many ways an approach to the secondary type of plate. There has usually been made a considerable difference between primary and secondary plates, but in reality no definite line of differentiation exists; both types grade into each other, and the primary plates are only a small...