A Non-Humanoid Alien

See:

Smokesmith
Faces
Aliens In Anderson And Niven
Rax

The Sigman is described in Poul Anderson's The Byworlder, IV. The description is complicated and somewhat unpleasant and I will not repeat it here. Of interest is that this quadrupedal ellipsoid has two arms at each end and eyes that can look in any direction and thus has neither front nor back. It has some features in common with Smokesmith.

Puppeteers are tripedal and two-headed and have their brains in their bodies, not enclosed in bone on the top;

Moties are asymmetrical, with one thick arm, two thin arms and only one ear;

the Sigman has neither front nor back.

These are imaginative aliens that can be characterized in a few phrases. All need:

limbs for locomotion and manipulation;
orifices for ingestion and excretion;
sense-organs;
a central nervous system.

In describing them, we refer to familiar body parts even though these are organized and distributed differently.