Living specimens are up to 11 mm long and 1.7 mm wide, but more commonly they measure 7 mm x 1.5 mm. This plump and robust species has two eyes set distant from the anterior margin. The head is usually bluntly triangular in shape, sometimes rounded. The dorsal surface is usually yellow-brown in colour, often with an olive tinge; the branches of the intestine that extend anteriorly to the eyes frequently show through the surface. The ventral surface is not pigmented and in mature specimens only one gonopore is present.
The penis is very large, often highly glandular, and usually bent posteriorly in the middle region; the vasa deferentia open separately into the penis bulb. The oviducts open into the glandular female genital duct (vagina and primary bursal canal), which extends posteriorly from the male atrium (F. affinis-detail); there are no bursa copulatrix or receptacular ducts.
This species is known from Scandinavia, Greenland and Atlantic North America.