Charles Messing's Crinoid Pages: Stalk and Centrodorsal
Stalk and Centrodorsal
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, 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 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. In Isocrinida, modified columnals called nodals that occur at intervals along the stalk each usually bear a whorl of five unbranched, usually hook-like or prehensile cirri composed of ossicles called cirrals. The terminal cirral is usually 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). 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 (Roux et al. 2002).
Some stalked crinoids (e.g., Hyocrinidae) cement to hard substrates via an expanded terminal holdfast 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. In some other genera (e.g., Rhizocrinidae), one or more distal columnals bear slender branched or unbranched radicles that together form a root-like radix for anchoring in unconsolidated substrates. Radicles have been called radicular cirri, but, unlike true cirri, they do not arise from well-defined sockets in columnals modified as nodals. Most species with cirri anchor either with those arising from the terminal nodal or those along a distal portion of stalk that lies along the seafloor. 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). See it in action here.
In genera with a reduced column (e.g., Neogymnocrinus), 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).
References
Améziane, N., Bourseau, J. P., Heinzeller, T. and Roux, M. 1999. Les genres Cyathidium et Holopus au sein des Cyrtocrinida (Crinoidea; Echinodermata). J. Nat. Hist. 33:439-470.
Baumiller, T.K. & C.G. Messing. 2005. Crawling in stalked crinoids: observations, functional morphology, and implications for Paleozoic taxa (Paper No. 25-11). Geol. Soc. Amer. Abstracts with Programs 37(7):62-63.
Bourseau, J.P., Améziane-Cominardi, N., Avocat, R. & Roux, M. 1991. Echinodermata : Les Crinoïdes pédonculés de Nouvelle-Calédonie. In: A. Crosnier, ed. Résultats des Campagnes Musorstom 8. Mém. Mus. natn. d’Hist. nat., Paris (A)151: 229-333.
Messing, C.G., RoseSmyth, M. C., Mailer, S.R. and Miller, J.E. 1988. Relocation movement in a stalked crinoid (Echinodermata). Bull. Mar. Sci. 42:480-487.
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.
CENTRODORSAL
Feather stars develop a stalk during their postlarval stages 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 is so named because this side of a feather star was formerly called dorsal. 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. Feather stars 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 feather star 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. Many other feather stars lose their apical cirri, and their sockets become 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 Comatulidae (formerly Comasteridae) (e.g., Phanogenia, Comanthus, Capillaster) bear a reduced, thin centrodorsal with few or no cirri. Most of these crinoids appear to develop a few cirri as juveniles but lose them with growth.
CIRRI
Cirri are mostly hook-like and often prehensile, with the terminal segment modified as a curved terminal claw and the preceding cirral 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, ridge, tubercle, pair of spines or tubercles, or a fringe of fine spines. [Modified from Roux et al. (2002) and Messing (1997)].
References
Messing, C.G. 1997. Living Comatulids. Pp. 3-30 IN: Waters, J.A. & Maples, C.G. (eds.) Geobiology of Echinoderms. Paleontological Society Papers 3.
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.