Arthracanthida

{{Short description|Group of protists}}

{{Taxobox

|

| name = Arthracanthida

| domain = Eukaryota

| unranked_regnum = SAR

| unranked_subregnum = Rhizaria

| phylum = Retaria

| classis = Acantharea

| subclassis = Arthracanthida

| subclassis_authority = Schewiakoff, 1926 {{cite web |author=WoRMS |year=2010 |title=Arthracanthida |publisher=World Register of Marine Species |url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=367301 |accessdate=July 17, 2011}}

| synonyms = Acanthometrida

}}

file:Radiolaria (Challenger) Plate 130.jpg

Arthracanthida, a subclass of Acantharea, is a group of marine protozoans. They consist mainly of a gelatinous sheath filled with cytoplasm and a skeleton of up to 20 radially placed spicules made of celestine (strontium sulfate). While mostly found in the upper areas of the ocean, they are able to move vertically by using microfilaments attached to the spicules to expand and contract the sheath. They are plentiful in the Gulf Stream during the summer months, but little is known about their overall distribution.