Aside from the water vascular system, echinoderms have a haemal coelom (or haemal system, the "haemal" being a misnomer), a perivisceral coelom, a gonadal coelom and often also a perihaemal coelom (or perihaemal system). Source: Internet
Because of their catch connective tissue, which can change rapidly from a flaccid to a rigid state, echinoderms are very difficult to dislodge from crevices. Source: Internet
Even at abyssal depths, where no light penetrates, synchronisation of reproductive activity in echinoderms is surprisingly frequent. citation Some echinoderms brood their eggs. Source: Internet
It seems probable that the mouth-upward orientation is the primitive state and that at some stage, all the classes of echinoderms except the crinoids reversed this to become mouth-downward. Source: Internet
Taxonomic affinity seeAlso The group had been variously assigned to the echinoderms, barnacles, annelids and mollusks, before the discovery of a fossil preserving soft tissue allowed a firm classification to the annelids, in 2008. Source: Internet
The body cavities of many sea cucumbers and some starfish provide a habitat for parasitic or symbiotic organisms including fish, crabs, worms and snails. citation Use by humans In 2010, 373,000 tonnes of echinoderms were harvested, mainly for consumption. Source: Internet