The name Echinoderm means spiny skin in Greek and refers to the numerous species of starfish and sea urchins which have spiny extensions of their skeleton. Echinoderms are unique in several ways: (1) it is group of organism entirely marine without any known parasite; (2) they have a complex internal skeleton made of calcium carbonate; (3) although mobile, they have a radial symmetry, typically of order 5 (4) they possess a complex system of water canals that move numerous tube feet for locomotion and feeding and (5) they have a mutable collagen than can change stiffness on demand.
Echinoderms are relatively large invertebrates and they are abundant in most ecosystems from the polar seas to the tropical coral reefs. The group is divided into 5 taxonomic classes with somewhat different morphologies: (1) Echinoidea (sea urchins), (2) Asteroidea (sea stars), (3) Holothuroidea (sea cucumbers), (4) Ophiuroidea (brittle stars) and (5) Crinoidea (feather stars). Members of all five classes are found in Oman.
Echinoderms are relatively large invertebrates and they are abundant in most ecosystems from the polar seas to the tropical coral reefs. The group is divided into 5 taxonomic classes with somewhat different morphologies: (1) Echinoidea (sea urchins), (2) Asteroidea (sea stars), (3) Holothuroidea (sea cucumbers), (4) Ophiuroidea (brittle stars) and (5) Crinoidea (feather stars). Members of all five classes are found in Oman.
All Echinoderms have an internal skeleton of calcium carbonate—limestone— porous plates secreted by cells in the dermis (the skin: hence the name Echinoderm).
- In regular sea urchins, these plates form a continuous, nearly spherical and rigid envelope: the test.
- In starfish, the plates are articulated and allow the starfish to change somewhat shape.
- In both crinoids and ophiuroids, the central disk is encased in a series of rigid plates but the arms are composed of numerous segments that gives these structures an amazing flexibility.
- Finally in sea cucumber, although present, the skeleton is reduced to microscopic spicules. For most groups, the taxonomy is based on the skeletal structures and it is only recently that behaviour, color, and of course genetics have contributed to a better understanding of the diversity of this large group of invertebrates.