Echinodormata
Echinoderms form a well-defined and highly-derived clade of metazoans. They have attracted much attention due to their extensive fossil record, ecological importance in the marine realm, intriguing adult morphology, unusual biomechanical properties, and experimentally manipulable embryos. The approximately 7,000 species of extant echinoderms fall into five well-defined clades: Crinoidea (sea lilies and feather stars), Ophiuroidea (basket stars and brittle stars), Asteroidea (starfishes), Echinoidea (sea urchins, sand dollars, and sea biscuits), and Holothuroidea (sea cucumbers). The phylogenetic position of the Concentricycloidea (sea daisies; 2 species), remains controversial
Starfish
In a starfish, the coelomic side of the body wall contains an outer circular and an inner longitudinal muscle layer. The longitudinal muscle is thickened into a median aboral line that runs from the disk along each arm. An upper transverse muscle and a lower transverse muscle connect each pair of ambulacral ossicles. Contraction of the upper causes the ambulacral groove to widen, whilst contraction of the lower narrows the groove. The upper and lower longitudinal ambulacral muscles connect adjacent ambulacral ossicles, contraction of which shortens the ambulacral groove. Longitudinal muscles between adjacent ambulacral ossicles aid in sideways movements of the arms. Dorsolateral arm muscles in Benthopectinids may cause thrashing movements of the arms allowing these starfish to swim.
Sea Lily
Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. Crinoids comprise three basic sections; the stem, the calyx, and the arms. A calyx is a cuplike structure on an animal. The calyx contains the crinoid's digestive and reproductive organs, and the mouth is located at the top of the dorsal cup. The arms display pentamerism or pentaradial symmetry and comprise smaller ossicles than the stem and are equipped with cirri which facilitate feeding by moving the organic media down the arm and into the mouth
Sea Cucumbers
Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles joined by connective tissue. In some species these can sometimes be enlarged to flattened plates, forming an armor.The body of a holothurian is roughly cylindrical. It is radially symmetrical along its longitudinal axis, and has weak bilateral symmetry transversely with a dorsal and a ventral surface. As in other Echinozoans, there are five ambulacra separated by five ambulacral grooves, the interambulacra. The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothurians, especially on the dorsal surface. The two dorsal abulacra make up the bivium while the three ventral ones are known as the trivium.