The crinoid crown consists of the theca and rays, and the theca consists of the calyx and tegmen. The calyx of extant crinoids consists of one, two or three circlets or rings of rigidly attached ossicles immediately above the stalk. It is often used synonymously with aboral (or dorsal) cup. However, the latter is better treated as a functional entity that may also incorporate proximal columnals and/or plates of the tegmen. Adjacent to the stalk, three to five basal ossicles form an aboral circlet. Five radial ossicles form a circlet above the basals, each associated with one of the five internal radial canals of the water vascular system that arise from the ring canal that encircles the esophagus. “Radial” is also an adjective: structures associated with the extrapolated central axis of these ossicles have a radial orientation. Because basal ossicles alternate with radials (the center of a basal ossicle usually lines up with the border between adjacent radials), their orientation is interradial. Distinct suture lines may demarcate both basals and radials, or they may be variously fused. A third series of calyx ossicles, the radially-oriented infrabasals, occur in some extant taxa between the basals and the stalk, but only as reduced internal elements.
In all extant comatulids (except the Atelecrinidae), the basal ossicles metamorphose during early development into a delicate internal rosette that roofs the centrodorsal cavity. Five narrow interradial rods or tongues (basal rays) may radiate from the rosette; they are sometimes visible as tubercles at the proximal corners of the radials (interradial angles). Because the basals are usually thus reduced, the centrodorsal lies directly against and at least partly covers the radial ring.
Calyx ossicles vary from thin-walled plates that form a hollow cup that encloses the viscera (above, left), to small wedge-shaped ossicles together little or no wider than the top of the stalk (above, right). Morphology, proportions and extent of fusion of calyx ossicles are often critical diagnostic features at subordinal, familial and generic levels.
The calyx may also be considered functionally as the support for the viscera, in which case it often incorporates additional ossicles (Ausich, 1996). In comatulids and isocrinids, for example, which account for the great majority of living crinoids, the calyx is not cuplike and no longer surrounds the visceral mass. Instead, the viscera rest on top of the radial circlet and proximal ossicles of the rays.
The oral surface of the theca, the tegmen (or disk), may be naked integument, invested with small calcareous pieces, or completely or partly covered with small plates or nodules. The disk bears both the anus, located at the apex of a small tube or papilla (AP), and bases of the ambulacral food grooves (AFG) that converge on the mouth (M). The mouth is usually located centrally on the tegmen with the anus displaced to one side. However, most comasterids, a chiefly reef-dwelling comatulid family, reverse this arrangement(right).
Each radial represents the first, or most proximal, ossicle of a ray. Arm (or brachium) and ray both refer to the usually branched series of ossicles and associated soft tissues that radiate from the central body. A ray begins with a radial ossicle whereas an arm begins with the first ossicle following a radial. Use of one versus the other has derived largely from morphology. In crinoids with a reduced calyx and with radials similar to the following ossicles (e.g., Isocrinidae, top right, and comatulids), the term ray is often used. In this context, arm refers to the unbranched series of ossicles following the most distal branching point. In crinoids with a well-developed calyx (e.g., Hyocrinus, top left), the term arm refers to the entire distinct, often much narrower, series of ossicles following the large radial.
Whether arm or ray is used, brachial ossicles follow the radials. Brachitaxes (or division series) are series of 2 to 20 brachials between branch points, either following a radial and including the first ossicle at which the ray branches (axillary or axil), or following an axil and including the next. Each axil bears two articular faces distally and may thus bear two additional brachitaxes, two unbranched arms, or one of each. Interior and exterior arms, brachitaxes or associated structures are those closest to and furthest from, respectively, the extrapolated axis of the preceding branching series. The first three brachitaxes, beginning immediately following the radials, are often specified as primibrachial, secundibrachial and tertibrachial series, composed of primi-, secundi- and tertibrach ossicles, respectively. Branching patterns are often diagnostic at generic and, sometimes, family levels.
Although some crinoids have unbranched rays, most start out with 10 arms (one fork per ray); additional arms develop when one is shed and two or more grow back in its place. Many species have 10 arms only; others have as many as 40 or 50; some reef-dwelling tropical species have as many as 250. Rays and their various branches carry extensions of coelomic, nervous and water vascular systems. Branching patterns are often diagnostic at generic and, sometimes, family levels. Two comatulid genera, Thaumatocrinus (Pentametrocrinidae) and Promachocrinus (Antedonidae), are exceptional in having 10 rays instead of five that arise from five radials alternating with five pararadials.
Pinnules are the small, segmented, unbranched appendages that arise on alternating sides of successive brachials, give the arms their characteristic featherlike appearance and are the primary site of food-collection. They are composed of ossicles called pinnulars. In comatulids, one or more pairs of oral pinnules near the arm base may be modified and lack an ambulacral groove. Longer and more flexible forms are modified ostensibly for sweeping and more robust and spine-like forms for protecting the oral surface. Several pairs of genital pinnules, which bear the gonads, follow. They are usually either similar to or shorter and stouter than the more distal, exclusively food-collecting pinnules, although in a few taxa the middle segments are broadened and roof the gonads. In several families, delicate side and covering plates border and can close over and protect the ciliated ambulacral food grooves on the pinnules and arms.
[Modified from Roux et al. (2002)]
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