The Crinoid, Encrinite or 'Sea Lily' is exclusively a marine animal. Though resembling a plant it is a member of the Phylum Echinodermata. The Crinoidea comprise of one of the Classes of the Subphylum, Pelmatozoa or attached forms (in contrast to the Eleutherozoa, the free and vagrant forms of which the starfish and sea-urchins belong). Also included in the Pelmatozoa are the Cystoidea of which the Blastoids are but one Class.
The Class Crinoidea consists of four sub-classes: the Inadunata, Flexibilia, Camerata and Articulata. Crinoids belonging to the first three subclasses first appeared in the Ordovician Period. The Flexibilia and the Camerata became extinct in the Permian and the Inadunata in the Triassic. The Articulata appeared first in the Triassic and continued to the present day.
Crinoids are typically characterised by their pentameral radial symmetry. The three main parts of the crinoid are shown in the diagram.
The Calyx: contains the vital organs of the animal. It is small when compared to the total mass, most of which is devoted to food collection. The mouth and anus are located on the upper surface of the body, and are connected by a simple gut.
The Arms: composed of an articulated series of ossicles that are used in suspension feeding and respiration. The gonads are also located in the arms; fertilisation takes place in open sea water during mass spawnings.
The Stem: this supports the animal and together with the roots and cirri served as a means of attachment to the sea bed or other useful substrate such as logs.
The ventral or oral surface of the animal is that part of the calyx above the base of the arms and is referred to as the Tegmen. The dorsal or aboral surface (the Dorsal Cup) lies below the arm base. The combination of the calyx and the arms are commonly known as the Crown.