Cratera assu is a species of land planarian belonging to the subfamily Geoplaninae.[1] It is known from specimens found in Serra da Bocaina National Park in Brazil.
Cratera assu is a flatworm around 50–65 mm in length and 3–3.5 mm in width. The body is elongate and slightly lanceolated, with a rounded front tip, and a pointed back tip. Widest at the pharynx, the body narrows towards the front and back. The dorsal side of the body can vary in color from chestnut brown to black-brown. In the front tenth of the body, there is a submarginal stripe that is yellow-orange or grayish in color; towards the back, this stripe is marginal. The submarginal stripe is inconspicuous in some individuals. Some individuals have a thin grayish midline. The ventral side of the body can range from sand yellow to light ivory in color.[2]
Aside from its coloration and length, it is distinguished from other members of Cratera by having dorsal eyes, a bell-shaped pharynx, a horizontal penis papilla, a widened distal portion of the ejaculatory duct that occupies most of the penis papilla, and the lack of a common glandular ovovitelline duct.
The specific epithet is derived from the Tupi language word, "large", in reference to the large size of the ejaculatory duct's distal dilation.