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A stalked crinoid (white) and a comatulid (red) in deep sea, showing the differences between these two sister groups. Most modern crinoids, i.e., the feather stars, are free-moving and lack a stem as adults. Examples of fossil crinoids that have been interpreted as free-swimming include Marsupites, Saccocoma and Uintacrinus. [23]
Articulata are a subclass or superorder within the class Crinoidea, including all living crinoid species. They are commonly known as sea lilies (stalked crinoids) or feather stars (unstalked crinoids). The Articulata are differentiated from the extinct subclasses by their lack of an anal plate in the adult stage and the presence of an ...
Crinoids are supported by jointed stalks containing substantial compound ossicles. The crown has ossicles scattered throughout the connective tissue (crinoids have no distinct dermis). The arms contain columns of well-developed vertebrae-like ossicles. Each joint has limited movement but the whole arm can be coiled and uncoiled. [9]
In epimorphosis stem cells, either from a reserve pool or those produced by dedifferentiation, form a blastema and generate new tissues. Morphallactic regeneration involves the movement and remodelling of existing tissues to replace lost parts. [52] Direct transdifferentiation of one type of tissue to another during tissue replacement is also ...
Uniquely among echinoderms, crinoids have no madreporite. Instead, the oral surface is dotted with numerous minute ciliated funnels that run into the main body cavity. The ring canal has several small stone canals, located between the arms of the animal, but these open into the body cavity, and thus are only indirectly connected to the outside.
The "beads" are thick discs or short cylinders, which, when the crinoid was still alive, were articulated to form a branched structure, linked by soft tissue, nerves and ligaments which occupied the central hole . The columnals usually became disarticulated after the animal died.
Pentacrinoidea includes most major crinoid groups, such as the living Articulata and extinct Flexibilia (together forming the parvclass Cladida) as well as the extinct Disparida. It is a stem-based taxon , defined as including all crinoids closer to Pentacrinites (a Jurassic articulate) or Apektocrinus (an Early Ordovician cladid ) than to ...
Stereom was the first form of biomineralization to evolve in deuterostomes, predating the evolution of spicules in tunicates and bone in vertebrates. [3] Stereom likely evolved before other distinctive traits of echinoderms, such as radial symmetry, as it is present in basal echinoderms with bilaterally symmetric or asymmetric body plans. [4]