If you wouldn't know you wouldn't suspect a Sea cucumber behind these tentacles. They are marine animals with a leathery skin and an elongated body containing a single, branched gonad.
The most common way to separate the subclasses is by looking at their oral tentacles. The shape of the tentacles is generally adapted to the diet, and to the size of the particles to be ingested: the filter-feeding species mostly have complex arborescent tentacles, intended to maximize the surface area available for filtering, while the species feeding on the substratum will more often need digitate tentacles to sort out the nutritional material.
Sea cucumbers are found on the sea floor worldwide. The number of holothurian species worldwide is about 1,717.