Echinoderms

Only marine organisms belong to the group of Echinoderms, whose species are distributed into five classes that include sessile organisms such as sea lilies (Crinoidea) and free organisms such as sea stars (Asteroidea), the brittle stars (Ophiuroidei), urchins (Echinoidea) and sea cucumbers (Holothuroidea). The majority of  them  live freely on the seabed.
Echinoderms show a radial pentameric symmetry, although some groups show a bilateral symmetry (sea cucumbers). They are characterised by the presence of a water vascular system used for walking, a lack of a cephalic region, a body organized around the oral-aboral axis, and the presence of a very developed dermaskeleton.
On a total of 143  Mediterranean species, 120 have been found in Italian waters.