The stalk arises from the center of the aboral surface of the theca and supports the body above and fixes it to the substrate. The central, supportive member is the column, composed of columnals (left), which are circular, pentagonal, stellate or elliptic (rarely hexagonal) in cross section and range from thin and discoid to tall and cylindrical. They may also be barrel-, spool- or hourglass-shaped. Columns range from a few centimeters to over a meter in length and may consist of over 350 columnals. Homeomorphic columns (right) consist of similar columnals that may change gradually along the length of the column. Heteromorphic columns consist of different kinds of columnals distributed along most or the entire column (e.g., nodals alternating with series of internodals, see below). In xenomorphic columns, the proximal, middle and distal sections of the stalk (proxistele, mesistele and dististele, respectively) each consist of different kinds of columnals. Each of these stalk sections may be either homeomorphic or heteromorphic.
In genera with a reduced column (e.g., Gymnocrinus), it is unclear if the remaining columnals derive from multiple fused ossicles and, if so, how many (Bourseau et al. 1991). Proximal columnals partly fuse to each other in some genera, and the proximal end of the column fuses completely with the calyx in others. Extant Cyathidium and Holopus appear to have suppressed the column completely and attach via the expanded aboral base of the calyx (Améziane et al. 1999).
Many stalked crinoids (e.g., Hyocrinus, Phrynocrinus) cement to hard substrates via an expanded terminal columnal, an attachment disk (right) that may encrust irregular substrates. All extant crinoids (perhaps excepting Holopus, Proeudesicrinus and Cyathidium) probably attach in this manner at least as postlarvae and, perhaps, juveniles.
Alternatively, the column may develop accessory anchoring structures. In isocrinids (also called pentacrinitids), modified columnals called nodals that occur at intervals along the stalk each bear a whorl of five cirri: unbranched, usually hook-like or prehensile appendages composed of ossicles called cirrals. The terminal cirral is often hooked and claw-like.
The series of non-cirrus-bearing internodal columnals between successive nodals is called an internode (and an internode plus the next distal nodal is a noditaxis). Most species anchor either via the cirri arising from the terminal nodal or those along a distal portion of stalk that lies recumbent along the substrate. These crinoids can release their cirri from the substrate, either partly in order to elevate or lower the crown, or completely in order to crawl with their arms (Messing et al. 1988, Baumiller & Messing 2005). In other genera (e.g., Democrinus, Bathycrinus), one or more distal columnals bear slender branched or unbranched radicles that together form a root-like radix for anchoring in unconsolidated substrates (above right). Radicles have been called radicular cirri, but, unlike true cirri, they do not arise from well-defined sockets in columnals modified as nodals.
Comatulids develop a stalk during their postlarval stages (left) but eventually shed it and retain only the topmost segment, a large modified ossicle called the centrodorsal, located in the middle of the aboral surface. (It may actually represent several fused ancestral stalk ossicles.) Centrodorsals come in a wide variety of shapes--disks, stars, domes, cones or cylinders--and usually bear numerous unbranched, segmented cirri for anchoring. Ligaments attach the cirri to sockets in the centrodorsal. All comatulids apparently develop five initial, radially-oriented cirri. As the centrodorsal enlarges, additional cirri are added at the margin in a double spiral pattern so that adjacent sockets are offset. Some species align cirri in columns (from apex to margin), with 2-3 columns under each radial often segregated by interradial ridges. However, the pattern of marginal cirrus addition remains the same. In some, small apical cirri developed when the comatulid was young are retained near the top of a conical or hemispherical centrodorsal. Alternatively, they may be lost, with traces of their attachment remaining as obsolete sockets. In both cases, however, at least a small area devoid of cirri and sockets, the aboral pole, remains at the centrodorsal apex. This area may be flat, convex or conical, smooth, papillose or spiny. In many other comatulids, apical cirri are lost and their sockets obliterated with centrodorsal growth so that the aboral pole broadens and cirri are restricted to one or two marginal rows. Several species in several genera of Comasteridae (e.g., Phanogenia, Comanthus, Capillaster) bear a reduced, thin centrodorsal with few or no cirri. Most appear to develop a few cirri as juveniles but lose them with growth.
Cirri are hooklike and often prehensile, with the terminal segment modified as a curved terminal claw and the penultimate bearing an opposing spine. They range from robust to slender and delicate and may have fewer than ten to more than one hundred segments. Cirrals typically increase in length from a short basal few to the longest, usually in the proximal half of the cirrus, which is followed by shorter distal segments. Alternatively, all cirrals may be uniformly short or, in a few species, those beyond the short basal few are long and straight, and the cirrus terminates in a straight tapered tip. The aboral (lower) surface may bear a spine, keel or ridge.
[Modified from Roux et al. (2002) and Messing (1997)]
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