Echinodermata (echinoderms), class: Holothuroidea (sea cucumbers)Collection Highlights | Updated 7 years ago Holothuria (Halodeima) atra Photo by Clay Bryce, image copyright WA Museum Holothurians are marine organisms with leathery skin and an elongated body. The holothurians are distinct within the echinoderms in having a circlet of flexible tentacles around their mouths called oral tentacles. These may be simple, digitate (finger-like); pinnate (feather-like), or peltate (flattened and shield-like). They also have a calcareous (made of calcium) ring that encircles their throat, serving as an attachment point for muscles operating their oral tentacles. Some holothurians can expel sticky tubules from their cloaca (rectum) which sometimes have toxins to entangle potential predators, some other animals (eg polychaete worms) also hide in the cloaca to hide from predators. In the very deep sea waters (>8.8km depth), holothurians make up more than 90% of macrofauna (visible fauna). Marine Invertebrates Section View the discussion thread.