Abstract
Urceolaria karyodactyla n.sp. occurs in the mantle cavity and on the ctenidia of its host, a circumpolar chiton. As seen in haematoxylin-stained slides it averages 65 μ in diameter and 60 μ in height, the diameter of the skeletal ring, which consists of 17–20 (usually 18) plates, averaging 23.5 μ. The macronucleus has an arm resembling a propeller blade on either side of a massive central body margined by several clubbed toe-like processes.