These are the sea cucumbers, Cucumaria sp. below right and Holothuna forskali (the cotton spinner) above.
There are just over 1100 living species, and they have an elongated bilaterally symmetrical soft body, and can grow to 50 cm long.
The skeleton has been reduced to small ossicles. The body muscles are attached to the ring of larger ossicles around the first part of the gut.
They have from 8 - 30 tentacles around the mouth operated by the water vascular system; these are used in feeding. They have a through gut.
Locomotion is mainly by tube feet, but in those species lacking tube feet, by peristaltic muscular contractions.
Respiration in most is via the anus. Water is drawn into special organs called respiratory trees. In small/thin species gaseous exchange takes place through the body wall.
Nutrition. Sea cucumbers live off the decaying matter found in the sand and mud they eat. Those species with longer tentacles also catch small organisms which stick to the slime covering the tentacles.
They are found mostly in or on the sea bed. Holothuna forskali is found in the N.E. Atlantic and Mediterranean, where it crawls on the sea floor like a slug using its tentacles to push mud into its mouth. It lives off the organic particles in the mud.
Defense. When disturbed or feeling threatened sea cucumbers shoot the tube of their respiratory system out of their anus. In extreme circumstances they can even shoot out their whole digestive tract. The tubes are sticky and entangle the potential predator allowing the sea cucumber to escape. The tubes and digestive tract can be re grown in weeks. When Holothuna forskali is threatened it moves its rear end in the direction of the aggressor and shoots out its respiratory trees through its anus. In contact with the water the respiratory trees swell up forming a sticky mass in which the aggressor gets entangled. New respiratory trees are soon regenerated.
Reproduction. The sexes are usually separate, though there are some hermaphrodite species, and fertilisation is external.