Sand dollars (species: Arachnoides placenta) in Lizard Island Field Guide (Lizard Island Field Guide)
Arachnoides placenta
Sand dollars

©Lyle Vail: Arachnoides placenta exposed at low tide at Watson's Bay beach - sand has been washed off

©Lyle Vail: Tracks of two Arachnoides placenta at low tide, Watson's Bay beach.

©Lyle Vail: Arachnoides placenta in its track with sand removed
Kingdom Animalia
Phylum Echinodermata
Class Echinoidea
Order Clypeasteroida
Family Clypeasteridae
Genus Arachnoides
Species Arachnoides placenta



Distinguishing features

Arachnoides placenta is a small sand dollar that grow to about 50 mm in diameter. It is flat underneath and only slightly raised on the upper side. The edge between the surfaces is very fine. It is usually covered with a thin layer of sand, leaving a track as it moves at low tide.


  • Size data has not been obtained.

Depth range

  • Depth range data is not yet available.



©Atlas of Living Australia: Australian distribution

Distribution and habitat preferences

At Lizard Island, Arachnoides placenta has so far only been found in the low intertidal zone at Watson's Bay beach where it is relatively common.

Web resources


References that assist with identification

  • Clark, A.M. and F.W.E. Rowe (1971). Monograph of shallow-water Indo-west Pacific echinoderms British Museum (Natural History), London.