Charles Messing's Crinoid Pages: Crown and Calyx
Crown and Calyx
The crinoid crown consists of the theca and arms (or rays), and the theca consists of the calyx and tegmen. The calyx of living crinoids consists of one, two or three circlets or rings of rigidly attached ossicles on top of the stalk. It is often used synonymously with aboral (or dorsal) cup. However, the latter term is better treated as a functional entity that may also incorporate proximal columnals and/or plates of the tegmen. Five basal ossicles (rarely three) form an aboral circlet at the top of the stalk. 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 (e.g., some Rhizocrinidae). A third series of ossicles, the radially-oriented infrabasals, occur in a few living species between the basals and the stalk, but only as reduced internal elements. They are better developed in many fossil species.
Calyx ossicles vary from thin-walled plates that form a hollow cup that encloses the viscera (e.g., Hyocrinidae), to small wedge-shaped ossicles together little or no wider than the top of the stalk (e.g., Isselicrinidae and Isocrinidae). Morphology, proportions and extent of fusion of calyx ossicles are often critical diagnostic features at subordinal, familial and generic levels.
In extant feather stars (except Atelecrinidae and Atopocrinidae) (Messing, 2013, 2020), the basal ossicles metamorphose during early development into a delicate internal rosette that roofs the centrodorsal cavity. Five interradial rods or tongues, referred to as basal rays, radiate from the rosette in many species; they are sometimes visible externally as tubercles at the proximal corners of the radials (interradial angles). Because the basals are usually thus reduced, the centrodorsal lies against and partly or completely covers the radial ring.
The calyx may also be considered functionally as the support for the visceral mass (also called the disk), which includes the digestive system, and components of nervous, coelomic, and hemal systems. In feather stars and most living stalked crinoids (except about 40, e.g., Hyocrinidae, Holopodidae), the calyx is not cuplike and does not surround the visceral mass. Instead, the visceral mass rests on top of the radial circlet and proximal ossicles of the arms (Ausich, 1996, 1997; Breimer, 1978; Hess et al. 2011).
References
Ausich, W. I. 1996. Echinodermata. Pp. 242-261, IN: Feldmann, R. M. (ed.) Fossils of Ohio. Ohio Division of Geological Survey, Bulletin 70.
Ausich, W. I. 1997. Calyx plate homologies and early evolutionary history of the Crinoidea. Pp. 289-304, IN: Waters, J. A. & Maples, C. G. (eds.) Geobiology of Echinoderms. Paleontological Society Papers 3.
Breimer, Albert. 1978. General Morphology Recent Crinoids. In Raymond C. Moore, & Kurt Teichert, eds., Treatise on Invertebrate Paleontology, Part T(2), vol. 1. Geological Society of America and University of Kansas Press; Boulder, Colorado and Lawrence, Kansas, p. T9–T58, figs. 1–40.
Hess, H., Ausich, W.I., Brett, C. E. & Simms, M.J. 1999. Fossil Crinoids. Cambridge Univ. Press, Cambridge. 275 pp.
Messing, C. G. 2013. A revision of the genus Atelecrinus PH Carpenter (Echinodermata: Crinoidea). Zootaxa 3681 (1): 001–043. http://dx.doi.org/10.11646/zootaxa.3681.1.1
Each radial represents the first, or most proximal, ossicle of a ray. The terms arm 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 and feather stars), the term ray is often used. In this context, arm may refer to the unbranched series of ossicles following the most distal branching point. In crinoids with a well-developed calyx (e.g., Hyocrinidae), the term arm refers to the entire distinct, often much narrower, series of ossicles following the large radial. Paleontologists use arm for the series of ossicles, branched or unbranched, following the radial, which is treated as part of the calyx.
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 (IBr), secundibrachial (IIBr), and tertibrachial (IIIBr) series, composed of primi-, secundi- and tertibrach ossicles, respectively, with additional series distinguished in succession as needed (e.g., IVBr and VBr). Branching patterns are often diagnostic at generic and, sometimes, family levels. Brachial shape may also be important in identifying species; most brachials are more or less wedge-shaped in aboral view, but they range from short and disk-like (like stacked thick coins) to elongated.
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 feather star genera, Thaumatocrinus (Pentametrocrinidae) and Promachocrinus (Antedonidae), are exceptional in having 10 rays instead of five that arise from five radials alternating with five pararadials.
References
Hess, H., Messing, C.G. 2011. Articulata. In Seldon, P. (Ed.) and W.I. Ausich (Coordinating Author). Treatise on Invertebrate Paleontology, Part T, Echinodermata 2 Revised, Crinoidea 3. University of Kansas Press, Lawrence, Kansas. xxix + 261 p.
Messing, C.G. 1997. Living Comatulids. Pp. 3-30 IN: Waters, J & Maples, C. (eds.) Geobiology of Echinoderms, Paleontology Society Papers, v. 3. Carnegie Mus. Nat. Hist., Pittsburgh.
Roux, M., Messing, C.G. & Améziane, N. 2002. Artificial keys to the genera of living stalked crinoids (Echinodermata). Bulletin of Marine Science 70(3):799-830.
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. It bears both the anus, located at the apex of a small tube or papilla, and the bases of the ambulacra (food grooves) that radiate from the mouth outward along the arms. In the great majority of living crinoids, the mouth lies in the center of the tegmen with the anus displaced to one side. However, in most Comatulidae (formerly Comasteridae), a chiefly reef-dwelling family of feather stars, the anal papilla is central, and the mouth displaced to near the edge of the oral surface, with the ambulacral grooves running around the periphery. In a few genera in this family, both mouth and anal papilla are displaced off center (=subcentral).
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 feather stars, one or more pairs of oral pinnules near the arm bases may be modified and may 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. Almost all crinoids have separate sexes, but the only way to distinguish them is to examine the gonads when they mature, and ovaries with eggs can be distinguished from testes. In stalked species and in several families of feather stars, delicate side and covering plates border and can close over and protect the ciliated ambulacral food grooves on the pinnules and arms (Messing, 1997; Roux et al., 2002). The way pinnules capture food is discussed under feeding mechanisms.
References
Messing, C.G. (1997) Living Comatulids. Pp. 3-30 IN: Waters, J & Maples, C. (eds.) Geobiology of Echinoderms, Paleontology Society Papers v. 3. Carnegie Mus. Nat. Hist., Pittsburgh.
Roux, M., Messing, C.G., Améziane, N. (2002). Artificial keys to the genera of living stalked crinoids (Echinodermata). Bulletin of Marine Science, 70(3):799-830.