Four orders of holothurians occur in northern Australia. The Order ASPIDOCHIROTIDA contains the largest and most conspicuous holothurians. They are typically sausage shaped animals with thick body walls. Aspidochirote holothurians ingest sand and digest some of the contained biota. The Order DENDROCHIROTIDA contains animals which are suspension feeders. They are often found with their bodies concealed, projecting their feeding tentacles into the water. The Orders APODIDA and MOLPADIDA contain animals lacking tube feet except for their modified feeding tentacles. The apodous holothurians are slender, vermiform animals with thin body walls. In the Family SYNAPTIDAE, species feed by gathering detritus from surfaces. Members of the Family CHIRIDOTIDAE engulf sand, i.e. their feeding is similar to that of aspidochirotes. The molpadid holothurians are sausage shaped animals with a thin body wall. Their bodies taper posteriorly to form what is called a caudal appendage. These animals burrow in sand and mud and feed there.
The absence of external characters which may be reliably preserved has led to a reliance for identification on the characteristic microscopic calcareous particles in the body wall. With practice, however, living animals often can be recognized in the field.

Habitat notes given must not be considered to exclude suitable habitats outside those types. Because of the great variability in the size of holothurians (often dependent upon the volume of water held within) sizes are given merely as a guide and the following length categories apply- small < 1OO mm, medium 100 - 300 mm, large > 300 mm.

Northern Australia shares its marine fauna with the rest of the Indo-west Pacific and numerous species recorded from this vast area stretching from the Red Sea to Tahiti that have not been recorded yet from Australia may well be found there. Reference should be made to the comprehensive account of Clark & Rowe (1971). Along the east and west coasts some overlap between tropical and temperate faunas can occur. For an account of the temperate forms from Southern Australia consult Rowe (1982).