Molpadida is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

s. The body shape is

fusiform Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a b ...

and unlike other sea cucumbers, their hind body is narrowed to form a distinct tail. Although they possess tentacles around the mouth derived from the

water vascular system The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms mov ...

, they have no true

tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...

, and are therefore believed to be related to the

Apodida Apodida is an order of littoral to deep-sea, largely infaunal holothurians, sea cucumbers. This order comprises three families, 32 genera and about 270 known species, called apodids, "without feet". Characteristics These sea cucumbers are vagil ...

. They have fifteen much branched tentacles and a stout body. They have tentacle ampullae and respiratory trees present, a kind of water lung attached to the cloaca. The

ossicles The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...

, minute calcareous plates embedded in the skin and characteristic of each species, take the form of tables, anchors, fusiform rods and perforated plates but never wheels.An illustrated key to the sea cucumbers of the South Atlantic Bight
Retrieved 2012-02-12. Like their relatives, they are adapted for burrowing into soft sediment, but they are relatively sedentary, rarely moving once they have excavated their burrow. Their burrow is U-shaped, with the animal's tentacles projecting from one end.

Notes

References

* Echinoderm orders Taxa named by Ernst Haeckel {{Holothuroidea-stub