Crinoids are pentamerous, stalked echinoderms with a cuplike body bearing five usually branched and commonly featherlike arms (see figure below). Most of a crinoid's body consists of an endoskeleton composed of numerous calcareous pieces, called plates or ossicles. The visceral mass of the crinoid animal is encased in the aboral cup that is typically composed of 2-3 circlets of plates. The mouth and anus are on the upper or oral surface of the animal. Additional circlets of fixed arm plates and fixed interradial plates may occur above the aboral cup, making a larger calyx. Five radial plates (the uppermost circlet of aboral cup plates) are aligned with the radial water vascular canals and give rise to five arms on the oral side of the body. Each arm is an articulated series of ossicles extending outward from the body. Arms contain extensions of coelomic, nervous, water vascular, and reproductive systems and bear anambulacral groove bordered by fingerlike tube feet, or podia (terminal extensions of the water vascular system), used in suspension feeding and respiration (see figure above). Arms may be nonbranching or branch in many different ways. All living crinoids are pinnulate, that is, they bear a small side branch (pinnule) on alternating sides of successive ossicles along the arm. In living crinoids, the pinnules bear the food-gathering tube feet. Pinnules arose in several lineages during the Paleozoic and are characteristic of all post-Palaeozoic crinoids.
General morphology of a stalked crinoid (modified from Bather, 1900, copyright © 1998 William Ausich)
The crinoid stalk typically consists of numerous discoidal skeletal pieces called columnals, held together by ligaments and penetrated by a central canal containing coelomic and neural tissue. In most species, the stalk serves to anchor the animal permanently to the substrate via one of a variety of terminal structures, e.g., a discoidal or encrusting holdfast, rootlike radix, or grapnel. In others, such as the living isocrinids, whorls of hooklike cirri (sing., cirrus) along the stalk allow the crinoid to release its hold and crawl with its arms. Several crinoid groups, notably the comatulids, which include the only living shallow-water crinoids, have lost the stalk. Comatulids anchor via numerous cirri that arise from the retained topmost columnal (the centrodorsal).
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