Echinoderms (Phylum Echinodermata) are characterised by their unique test (skeleton) which is made up of a number of interlocking plates, each comprised of a single calcite crystal with a complex 3D network, forming a porous structure, known as the stereom. All echinoderms are fully marine and display pentameral symmetry.
Their water vascular system, which forces water around the body using muscular action (a hydraulic system), is also unique to the group and is used in respiration, locomotion and feeding. Geologically, echinoderms are important as rock builders and some limestones may be made up exclusively of echinoderm remains (eg. crinoidal limestone).
Crinoids are either sessile, attached to the substrate by a skeletal column, or they are planktonic. Planktonic crinoids start life as sessile creatures and sever the connection as a secondary development. The bulk of the animal is the calcareous endoskelton with only a small proportion of living tissue. The skeleton is made up of numerous calcareous plates which, under magnification, can be seen to be highly porous. During life, these pores are filled with soft tissues, for example, muscle. The crinoid body is composed of three basic structural elements, the stem, the calyx and the arms.
The stem is segmented and composed of a number of circular (although some may be pentameral or elliptical) ossicles, held together by soft tissues. There is a hole running through the centre of the stem containing the coelom and the nervous system. In some crinoids, small rootlike projections from the base of the stem are used to root the crinoid into soft substrate or cement it to a hard one.
The calyx is built of two rings of interconnected plates forming a five-rayed symmetry. On the upper side of the cup, the mouth is situated in the centre and the anus on the periphery. The arms, or brachials, extend out from the calyx, usually comprising five branching structures. These brachials are also composed of ossicles and have cilia, which gather food and pass it down the arms to the mouth.
All crinoids are passive suspension feeders, which is a relatively ancient way of feeding that is energy efficient. This method produces no respiratory or feeding current, but they rely on ambient water movements. The cilia on the brachials are terminal extensions of the water vascular system.
The cilia or 'tubed feet' are grouped in threes, where the longest of the three is at right angles to a groove that runs down the centre of the arm. This tubed foot catches food which it then passes into the groove. The shortest tubed foot covers the food in a mucus, then passes it down the centre of the groove via a ciliary tract.
Crinoids eat a range of microscopic particles, ranging from diatoms, foraminifera, unicellular algae, small crustaceans, larvae and detritus. In some crinoids the vascular system also aids in movement, allowing the crinoid to 'walk' across the sediment substrate.