Charles Messing's Crinoid Pages: Introduction
Introduction
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The “classic” crinoid consists of a segmented stalk that supports a small central body, or theca, from which five, usually branched, arms (also called rays) radiate. Theca and rays together form the crown. The theca consists of a calyx (or aboral cup) that encloses or supports the viscera (often called the disk), and an overlying, sometimes-calcified membrane (tegmen) that bears mouth, anus and hydropores (tiny openings leading into the water vascular system) and defines the oral surface. In groups with a reduced calyx, the viscera rest on the bases of the rays. Five ambulacra radiate from the central mouth across the tegmen onto the rays and their branches. Each consists of a ciliated food groove lined with fingerlike, food-collecting podia (or tube feet; extensions of the water vascular system) and folds of epidermis. In orienting parts of a crinoid, distal describes a direction or position away from the central body, toward the tip of a structure (e.g., ray) or toward the anchoring end of the stalk. Proximal is toward the center, the base of a structure, or the upper end of the stalk. Structures associated with the surface opposite the mouth are aboral. Abambulacral is away from or opposite the surface bearing the food groove (=aboral). Adambulacral is toward the surface bearing the food groove. Adoral refers to a position toward the oral surface. |
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Skeleton
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Most of a crinoid’s body (usually at least ~80%) is a mesodermal endoskeleton that consists chiefly of articulated series of calcareous pieces (ossicles) held together by ligaments (and in some cases muscles). An axial canal carrying extensions of coelomic and nervous systems passes through each ossicle. As in other echinoderms, the fine structure of the ossicles forms a fine meshwork (stereom). The skeleton essentially determines body shape and forms the basis of much of crinoid taxonomy. This skeleton explains both why crinoids make good fossils (CaCO3 is basically limestone) and why not too many creatures subsist on a crinoid diet (too crunchy). Fortunately for taxonomists, it is usually covered only with a thin epidermis and is clearly visible, although you generally need a dissecting microscope to see many diagnostic features. With few exceptions, crinoid soft parts are not especially important in classification, but this may be because they’re a pain to deal with and few people have bothered. We’ll discuss visceral matters later. |