- Lack arms.
- Bilaterally symmetrical.
- Body wall soft rather than calcareous.
- Dioecious with a single gonad.
- Sedimentary feeders.
- Body surrounded by tube feet.
- Interior madreporite.
- Branched tentacles surrounding mouth that are lined with modified
water vascular system.
The holothuroideans, commonly called sea
cucumbers, are a very unique group of echinoderms. They are the only
class that is soft bodied rather than hard with well-developed
circular and longitudinal muscles. Their axis of symmetry is
horizontal unlike other echinoderms. The mouth and anus are located at separate ends of the sea cucumber's elongated body. The mouth is
surrounded by modified tube feet that form a ring of tentacles. The
number of tentacles varies from 8-30 depending on the order. This is one
feature that is used to divide this class into smaller groups. Tube feet
can be found all over the holothuroidean's body that are usually
contracted within the external body wall. Sea cucumbers are sessile
and very sluggish. They ingest large amounts of sand and filter out what
is not used. Both eggs and sperm are ejected into the water where
fertilization takes place. The body wall contains reduced spicules found
in shapes of rods, crosses or hooks. The respiratory system takes up
most of its internal mass and is found in no other echinoderm.
Some Interesting Facts:
- Sea cucumbers have the ability to eviscerate parts of, or its entire internal mass to ward off prey. In a least two species, autoeviscerationoccurs on a seasonal basis, but it is not known exactly why.
- Sea cucumbers lack arms unlike other echinoderms. Tentacles
have taken their place.
- Respiratory trees evolved and are found in no other echinoderm. These trees are outgrowths of the cloaca.
- Sea cucumbers can live from 5-10 years.
- They are found in all seas at all depths. Some ocean trenches contain more than 90% of the total biomass.
- Sea cucumbers are considered a delicacy in many countries around the world.
Pertinent References in Lamson Library:
- Barnes, R.S.K., Calow, P., Olive, P.J.W. 1988. The
Invertebrates: a new synthesis. Oxford: Blackwell
- Nichols, David. 1962. Echinoderms. London: Hutchinson
- Pawson, David L. 1982. Synopsis and Classification of Living Organisms New York: McGraw Hill Book Company.
(Carianne K. Meyers, Fall 1994; edited by T. Shultz, Spring 1995)