Paramoeba invadens n. sp. (Amoebida, Paramoebidae), a Pathogenic Amoeba from the Sea Urchin, Strongylocentrotus droebachiensis, in Eastern Canada1

Paramoeba invadens n. sp. is described from sea urchin tissues and from culture. Amoebae are 20–40 μm in length with an irregular hyaline region producing short subpseudopodia. One parasome per cell is present. The parasome is bipolar with basophilic, Feulgen‐positive poles and a Feulgen‐negative median segment. The fine structure of the parasome resembles those in other species of Paramoeba and the surface of the amoeba is plain with no hairs or scales. Amoebae dislodged from tissues often adopt a semifloating form; floating forms have been seen in culture. Amoebae circulate within lumina of the water vascular system and can migrate readily into or out of tissues.