Arachnactidae

{{Short description|Family of sea anemones}}

{{Distinguish|Penicillaria (moth)|Pennisetum}}

{{Automatic taxobox

| image = BEP2 2462-1.jpg

| image_caption = The ceriantharian Arachnanthus sarsi, Rathlin Island, Northern Ireland.

| parent_authority = den Hartog, 1977

| taxon = Arachnactidae

| authority = McMurrich, 1910

| subdivision = {{center|See text}}

}}

Arachnactidae is a family of tube-dwelling anemones in the order Ceriantharia. It is the only family in the monotypic order Penicillaria and comprises around 38 species. They differ from other ceriantharians in the makeup of their cnidome (the types of cnidocyte present), the relative sizes of the oral discs and the shape and structure of the mesenteries. These tube anemones dwell in parchment-like tubes immersed in soft sediment, and have two whorls of tentacles, the outer ones being much longer than the inner ones.{{cite book|author1=Goffredo, Stefano|author2=Dubinsky, Zvy|title=The Cnidaria, Past, Present and Future: The world of Medusa and her sisters|url=https://books.google.com/books?id=RdYADQAAQBAJ&pg=PA66 |year= 2016 |publisher=Springer International Publishing |isbn=978-3-319-31305-4 |page=66}}