Echinoderms (Phylum Echinodermata) are a phylum of marine animals.The phylum contains about 7,000 living species including sea cucumbers, sea urchins and starfish. The echinoderms are important both biologically and geologically: biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Many echinoderms have remarkable powers of regeneration. Some starfish are capable of regenerating lost arms. In some cases, lost arms have been observed to regenerate a second complete sea star. Sea cucumbers often discharge parts of their internal organs if they perceive danger. The discharged organs and tissues are quickly regenerated. Sea urchins are constantly losing their spines through damage — all parts are replaceable. Some starfish populations can reproduce entirely asexually purely by the shedding of arms for long periods of time.