The Class Holothuroidea is the class of echinoderms containing the sea cucumbers or holothurians. Sea cucumbers are basically cylindrical animals with a horizontal axis of radial symmetry. They have a mouth and anus at opposite ends of the body. In contrast to other echinoderms the body wall is soft because the external skeleton is reduced to mostly microscopic ossicles. Tube-feet only occur scattered over the body and in a ring around the mouth, where they are modified into tentacles. As a result of their penta-radial, horizontal symmetry the body of holothurians consists of five raddi running from mouth to anus. The animal lies on the substrate with three ventral radii, the two dorsal radii facing upwards. Each radius contains a longitudinal muscle, a nerve-strand, and a water-vascular canal. There is an extensive coelom. The gut consists of an esophagus, stomach, intestine, and rectum. The stomach and intestine are often indistinguishable and the latter describes a loop before running to the rectum and anus.
The anterior portion of the esophagus is surrounded by a calcareous ring, providing support, as well as by a water-vascular ring. The latter gives rise to radial vessels which lead to the tentacles around the mouth opening and to other parts of the body.
The gonad consists of a single group of genital tubules, from which runs a single gonoduct towards the anterior end to open near the ring of tentacles. Sea cucumbers are dioecious, i.e. they have both male and female gonads. Eggs and sperm are expelled into the sea water, where fertilization takes place. Some groups of holothurians have planktonic larvae, whereas others lack such larval stages.
Holothurians feed by capturing small organisms with their tentacles, which are then stuffed into the mouth and food particles are wiped off as the tentacles are pulled out of the mouth. Other forms ingest quantities of the substrate from which they extract organic matter.
Most holothurians are secretive animals, only moving in a very sluggish manner. However, some are more active and deep sea species can swim actively.
Sea cucumbers, which may live for 5 - 10 years, are found in all seas and at all depths. Some species can have a very wide distribution. The greatest diversity of forms is found in shallow waters in the tropics.