The Actinosphaerium is a protist (protozoan) and belongs to the Phyllum Sarcodina.

It looks like a sea urchin and is from 200-1000 micrometers wide. This is quite large for a protist.

The image below (left) shows a close up of the endoplasm and ectoplasm, the water expelling vesicles (WEV) and how this protist uses the radial arms to move by flowing the protoplasm into the arms.

These are also called Heliozoans, or "Sun Animals". The body is spherical with stiff unbranched arms radiating in all directions.