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Cincinnatian crinoids are pelmatozoans, a grade of echinoderm characterized by their stalked mode of attachment. Like other echinoderms, crinoids are composed of individual calcite pieces called ossicles, which are held together by muscles, ligaments and other soft tissues. A listing of crinoid anatomical terms is presented below. This listing is not intended as a comprehensive collection of crinoid morphological terms but as a simplified guide to assist the reader in understanding the crinoid descriptions.
Anal sac – A tall sack-like structure that elevates the excretory function away from the feeding appendages. Anal sacs may be smooth or highly ornamented, and are made of dozens to hundreds of ossicles.
Anal series – Calyx plates originating at the radials that extend upward leading to the anus.
Anal X – A plate in the cup of certain crinoids between radial plates from which the anal series extends.
Arm – Arms support the feeding appendages of the crinoid. These may or may not branch above the top of the calyx. Arms have grooves along their interior surface that in life had cilia to transport food particles to the mouth of the animal. Some groups of crinoids had pinnules along the arms to add additional surface for filtering food out of the water.
Basal – Plates found in the cup of crinoids immediately beneath the radial plates. Their position is generally rotated 36 degrees from the radial plates.
Calyx – The part of the crinoid body between the top of the stem and the base of the free arms.
Cirri – A small diameter jointed stem-like appendage emanating radially from the column.
Column – The support structure resembling the stem or stalk of a plant that supports the crinoid above the substrate. The column is comprised of numerous individual ossicles called columnals. The terms column, stem and stalk are used interchangeably in the descriptions.
Columnal – A single ossicle of the column.
Crown – The entire body of the crinoid excluding the column. This includes the cup, calyx and arms.
Cup – A common synonym for the calyx.
Discoidal holdfast – A multiplated convex disc known locally as the form genus “Lichenocrinus.” This is the attachment structure for many crinoids in the Cincinnatian and many morphological variations of this form exist. The different forms cannot always be associated with a specific crinoid species.
Distal – The end of a crinoid's appendage that is farthest from the calyx. For stalks, this is the holdfast end; for arms, this is the tip of the free arm.
Distal coils – A type of holdfast for many crinoids in which the distal end of the column has grown around another object for support.
Free arms – Extension of the rays that extend above the calyx.
Holdfast – Any structure that serves to anchor the crinoid to the substrate.
Infrabasals – These are ossicles in the calyx that form a circlet below the basals in some crinoids. Crinoids with this configuration will have infrabasals topped by basals which are in turn topped by the radials. Each circlet is offset by 36 degrees from the other.
Internodals – Column ossicles of smaller diameter than and situated in between the nodals are called internodals. These can vary in number and diameter.
Interray – Calyx plates situated between the rays.
Mesistele – The middle portion of the column between the proximal and distal ends.
Nodal – Column ossicles of larger diameter than the rest of the column. Nodals are spaced along the column separated by varying numbers of internodals.
Ossicle – An individual calcite piece of the crinoid skeleton.
Pentameric – A descriptive term for a polymeric columnal made from five individual ossicles.
Pinnule – A small diameter, regularly arranged extension from the arm that increases the crinoid’s food gathering capability. Crinoid arms with pinnules resemble a bird’s feather. In the Cincinnatian, crinoids having pinnules are typically those in the subclass Camerata.
Polymeric – A descriptive term for a columnal made from many individual ossicles.
Plate – A thin flat ossicle typically used to refer to those comprising the calyx.
Proximal – The end of a crinoid's appendage closest to the calyx. For the stem, this is the end immediately beneath the calyx; for arms, this is the end that attaches to the top of the calyx.
Radial – Calyx plate immediately above the basal circlet that supports the arms or the base of the rays of the calyx.
Radicular cirri – Cirri that function as a root-like anchor.
Ramule – Minor arm branch that differs from pinnules by its less regular frequency and, in some crinoids, that it has pinnules emanating from it.
Ray – Descriptive term for the radial plate and all subsequent plates continuing above it.
Tegmen – A covering for the top of the calyx, almost as a lid covers the top of a cup. The tegmen may or may not be constituted of calcified ossicles. The mouth and anal structure are a part of the tegmen.
Trimeric – A descriptive term for a columnal that is made from three individual ossicles.
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