Apodida 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 ...

of littoral to deep-sea, largely infaunal holothurians, sea cucumbers. This order comprises three families, 32

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

and about 270 known

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

, called apodids, "without feet".

Characteristics

These sea cucumbers are vagile holothurians with an elongated shape (up to 3 meters for '' Synapta maculata''), worm or snake-like. Their shape is adapted for burrowing through the sediment, sometimes in a fashion similar to

earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. ...

s. Their mouth is surrounded with 10-25 pinnate or peltate tentacles. The absence of tube feet gives the order its name, ''Apodida'' meaning ''without feet'' : they move by crawling on the sediment, hence they need flat bottoms with few current. Members of this order have a circum-oral ring and

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s, but do not have

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 bri ...

or radial canals. They also lack the complex respiratory trees found in other sea cucumbers, and respire and excrete nitrogenous waste through their skin. The ossicles, minute calcareous plates embedded in the skin and characteristic of each species, can include wheel and anchor shapes.An illustrated key to the sea cucumbers of the South Atlantic Bight
Retrieved 2012-02-12.

List of families

This order contains 3 families, consisting in 32 genera in total, for 270 known species. * family Chiridotidae Östergren, 1898 * family Myriotrochidae Théel, 1877 * family

Synaptidae Synaptidae is a family of sea cucumbers that have no tube feet, tentacle ampullae, retractor muscles, respiratory trees, or cuvierian tubules. They also lack radial canals of the water-vascular system, with only the circumoral ring present. S ...

Burmeister, 1837 File:Chiridota heheva.jpg, '' Chiridota heheva'' File:Polycheira rufescens.JPG, '' Polycheira rufescens'' File:Euapta godeffroi.jpg, '' Euapta godeffroyi'' File:Euapta lappa.jpg, '' Euapta lappa'' File:Polyplectana kefersteinii.jpg, '' Polyplectana kefersteinii'' File:Synapta maculata 2.jpg, '' Synapta maculata'' File:Synaptula lamperti (Sea cucumber).jpg, '' Synaptula lamperti''

References

Echinoderm orders {{Holothuroidea-stub