The Echinoid Directory

Sand dollars have a unique arrangement of tube-feet and ambulacral pores. Whereas other echinoids have just a single tube-foot arising from each ambulacral plate, in sand dollars there are tens or hundreds of individual tube feet to a plate. These are very small and end in a sticky suckered disc. They do not have individual ampullae internally but emerge directly from the lateral branches of the radial water vessel. Their primary function is in food gathering and their small size allows them to specialise in capturing the fine organic particles adhering to sand grains.

There are also aboral leaf-like tube-feet specialised for gaseous exchange as in other irregular echinoids. These emerge from the slit-like pore-pairs that form the aboral petals.

The photograph above shows the spines (sp) and tube-feet (tf) of the sand dollar Encope in close-up. All the tube-feet visible come from a single ambulacral plate