Cygnan

Cygnans are a sapient species which evolved in a moon of a gas giant orbiting a binary star system in the Cygnus Constellation. Knowing that one of their suns was about to go supernova, the Cygnans moved their entire civilization to vast spaceships and abandoned their home system, taking their gas giant with them as a source of fuel.

The interior of their ships is filled with trees, recreating the Cygnans' natural forest environment, complete with numerous species of small creatures that they feed upon. Their civilization is over six million years old.

Biology
The Cygnan species has an extreme form of sexual dimorphism in which only the females are sapient. The Cygnan males are tick-like non-sapient parasites, about the size of a mouse, which cling to the abdomens of their sisters.

Female Cygnans are a bit smaller than Humans and have elongated cylindrical bodies with sleek mottled skin. They have narrow heads, six limbs and a three-petaled tail, although when the petals are closed the tail appears to be club-shaped. Their limbs protrude directly from the body, without shoulders, and all end in tridactyl hands or feet. The middle limbs can be used as either arms or legs, allowing the Cygnans to swiftly alternate between bipedal and quadrupedal stance. On rarer occasions, they can also walk on all six limbs. They have three eyes on mobile stalks, two of which are located above the mouth and the third below it. The head is shaped like an arrowhead, with a toothless mouth that seems to smile permanently. Although they have no teeth, the interior of the mouth is textured like a rasp and also equipped with a tubular spiked tongue. Their diet is carnivorous. They feed on small creatures which are placed alive inside the Cygnan's mouth and sucked out of their internal fluids before being swallowed whole.

Cygnan females move quickly in short bursts. Having evolved in a world with roughly one third of Earth's gravity, they are not nearly as strong as Humans, but are able to compensate that with agility and the dexterity provided by their six limbs. Their cartilaginous skull allows their heads to bend flexibly. Compared to Humans, they prefer slightly colder temperatures. Their internal fluids are orange.

It's been speculated that Cygnans may have evolved from hydrozoan-like ancestors with radial symmetry, with a nerve cord running centrally inside their bodies (as opposed to dorsally or ventrally) and a brain located inside the upper torso. The asymmetrical arrangement of their limbs suggests that they used to have two sets of three limbs, but evolved to three sets of two by a process similar to a flatfish's acquired asymmetry. Over the couse of their evolution, one limb from the upper set and one from the lower set migrated towards the center of the body to form the middle pair.

Cygnans have multiple larynges and communicate in a musical language, in which tones are equivalent to phonemes. Females form monogamic pairs with other females, and reproduce by exchanging their parasitic males to fertilize each other.

Appearances

 * The Jupiter Theft, by Donald Moffitt (1977)