Tholians (Second Human Empire: Si'euji Sizz'kunhi) are a race from the planet Tholia, capital of the Tholian Assembly. They are known to be extremely xenophobic, rarely leaving their core territory. Tholians are mostly made of a crystalline material. Their biology requires high temperatures around 480 Kelvin (207 °C, 404 °F) in order to keep their hard mineral carapaces sufficiently soft. Although they could tolerate lower temperatures for a brief period of time, Tholians are killed by sustained insufficient heat as their carapace would begin to freeze and crack below ca. 380 Kelvin. They are independent in the timeline
A living Tholian was approximately the same height as a Human. It was principally a reddish color, in varying shades, and there was the suggestion of fluid or field motion within the body. Tholians had a hard carapace that was chiefly mineral. There were six thin legs that made it possible for the being to move in any direction quickly; the legs were articulated with joints roughly corresponding to the knee and ankle of humanoid species. Each leg ended in a multi-toed foot. The legs were attached at the base of the torso and were radially symmetric. There were two arms; each had joints analogous to the humanoid elbow and wrist, and each arm ended in a multi-fingered hand. There was a slightly visible neck that was covered by a semitransparent dome membrane that shielded the head. Tholians had two glowing spots near the top of their torso. They turned these to face individuals with whom they interacted, which suggested they were some sort of information-gathering organ(s). Tholians communicated primarily through a series of clicks and chirps. Tholians were hermaphroditic, with each individual possessing both male and female sexual organs.