The Xem is a small subterranean creature, about the size of a hat. It feeds on rotting tree roots between 4 and 20 feet underground.
It has a coat of short fur and a small mane around its head -- all of which is dingy black-brown. However, under black light the creature fluoresces a brilliant blue. This makes it much easier to spot, if one is using a sub-surface periscope.
The Xem has four two-fingered limbs and no tail. It is slow-moving, but sure-footed. The round head has two enormous eyes, but no nose or mouth for breathing. Its metabolism is almost entirely anaerobic.
The Xem's jaws are located in its midsection. The creature's ribcage is hinged at the spine; it has rows of tiny teeth where most animals would have a sternum. As the body swells and contracts, chewing is easily mistaken for breathing.
Xems live in small colonies of a dozen or so creatures, which move through underground tunnels between the trees that they depend upon for food. The adult Xem's fingers are not suited for digging; all of the tunnel systems are created by the young.
Infants are born in litters of 30 or more. At birth, the creatures have tough webbing between their fingers, ideal for excavation. Unlike their slow-moving elders, the young move about at a rapid pace, carving out living spaces for the next generation.
For unknown reasons, Xems have extremely high infant mortality rates: almost 90% of the young die within a month of birth. The bodies are transported to the lowest room in a tunnel system and left in piles to decay.
The sacrifice of the young does seem to serve an evolutionary purpose: many workers are needed to create living spaces -- but there are not enough tree roots to support such a large population. It has been speculated that a minority of Xems are born with a slower metabolism -- and that these ones are the few who survive into adulthood, while the rest simply burn out.