Medusan
The Medusans were a highly intelligent telepathic and empathic non-corporeal race. Their thoughts were the most sublime in the galaxy, but their physical appearance was exactly the opposite. The sight of a Medusan rendered any corporeal viewer mad, and soon afterwards could cause death by massive organ failure, although some species with telepathic aptitude may be affected differently. Even viewing the reflection of a Medusan could cause catatonia and memory loss, though not permanent harm. Vulcans were capable of viewing Medusans, but only with the use of a specially filtered visor; upon direct exposure, it could induce a form of involuntary mind meld between the Medusan and Vulcan. For these reasons, Medusans usually used an opaque carrier pod when interacting with other species. The Medusans were renowned for their navigational abilities. (; )

As energy-based lifeforms, Medusans had no physical body. They were known to identify as genderless. Individuals lived together in hive minds. ()

Medusans made use of starships similar in configuration to Federation vessels. One Medusan, Kollos, was brought aboard the  as an ambassador to help with that very purpose. ()

In 2383, a Medusan known as Zero cobbled together a containment suit unassisted, despite their non-corporeal nature, so that they could interact with corporeals. They later helped commandeer an abandoned Federation starship, the , from the mining colony of Tars Lamora. They were being held captive there, being used as a weapon to drive unwilling prisoners mad. ()

Sometime prior to 2399, Cristóbal Rios of La Sirena deleted Medusan navigational techniques from the ship's Emergency Navigational Hologram, Enoch. ()

Medusans could be affected by certain telepathic spores. Those produced on the surface of Murder Planet had the same effect on them as any corporeal lifeform. ()

List of Medusans 
 Kollos
 Zero

Appendices

Appearances 
 
  Every episode

Background information 
The Medusans were named for the Greek mythological character of "Medusa", one of the three Gorgon sisters. The Gorgons were said to have such a frightful appearance that gazing upon them would turn living beings to stone.

According to the story outline and script of the episode, their name was spelled "Medeusan". 

A planet was seen on screen in the original "Is There in Truth No Beauty?", but the remastered version of the episode, a Medusan vessel was seen instead.

Apocrypha 
According to the reference book The Worlds of the Federation (p. 60), Medusans are native to the Venus-like planet Medusa, also known as Visalayan by the native Medusans. It is the fourth planet orbiting Xi Hydrae. Contact between the Medusans and the Federation was also extremely limited.

The non-canon Star Trek: The Next Generation Officer's Manual spells it "Medusan". The Officer's Manual also states that Medusans were permitted to serve in Starfleet as emergency navigators on board  starships. Such Medusans were honorary lieutenant commanders and had special quarters off an emergency navigation center, which kept them isolated from the rest of the crew.

A Medusan character named Korgan appears in the A Time to... novel series who was one of only twelve active Medusans in Starfleet as of the late 2370s.

External links 
 
 

de:Meduser
ja:メデューサン
Species
Non-corporeal species
Telepathic species