What did the true form of the aliens look like?

Their true form is briefly seen in the episode, Genesis. One of the aliens, "Dr. Lanier", has accidentally reverted to his native form. Creepily, he is being kept in an old fashioned diver's suit with the "bubble" helmet. Eventually, he is carried on a stretcher into a large tank of salinated water, where he is zapped by electricity, in which process he begins to return to human form.

The director cleverly does not permit us to see the Doctor's true form for very long or very clearly. Note: this is a FULL reversion, not the partial reversion featured in a couple other episodes, where the incompletely-reverted alien still retains humanoid-bipedal form.

I can't really describe the alien's native form - but last time I checked, Genesis was on You Tube, so you might catch it there.

To me the creature looks like a central torso with clubby proto-fins at the "bottom" and a bicephalous structure at the "top". It does not look capable of maneuvering on its own. Perhaps on the home planet the aliens evolved superior mental capacities and were able to direct other creatures to do their moving for them, including building spacecraft, and of course, the shape-morphing technology that enables them to create their human look.

One thing I can say about their original form: It is totally alien, small, maybe three feet long, with no ornate external organs (except the "head" and "fins"). They probably just threw something together in a hurry, but the thing's lack of specificity makes it seem utterly alien.

Please consult my thread below on Genesis: Pure Horror:

The Doctor reverts - for the first and only time actually seen, though briefly on this show - to his native form. Other episoded only show partial reversion, which is depicted as massive skin deformation, but Genesis is the only episode that assays to reveal the aliens' true shape.

The Doctor has been kept in an old diving suit immersed in a pool of water. Completely creepy. The suit, obviously empty of anything human, is lifted out of the pool. One of the aliens opens the helmet and tells the "doctor" that the regeneration process will start soon. The camera shows nothing but the gaping black void of the inside of the helmet - where a human head should be, but is not.

[Originally, this was accompanied by a shrill cry from the Doctor which I believe was edited out of the soundtrack for some reason. The Doctor also squeaked in the initial scene where he is being transported by two teen surfer aliens in the back of a station wagon. A cop stops them, hears the (edited out) cry and demands to see what's in back - to his everlasting loss.]

Then there's the regeneration scene itself, which shows the alien "doctor" in its native form. It is brought into a lab by two Invaders, carrying it in a stretcher. As they carry it to the top of a salinated water tank, the canvas bulges suggestively, tantalizingly. Then they drop the Doctor into the tank. Bubbles obscure his form, but what can be seen of him is truly alien, unexpected, unidentifiable and weird. There is a hint of bilateral symmetry and an intimation of possible bicephalacy. The camera does not linger. As the process proceeds, the Doctor by degrees begins to take on a human shape.

This is The Invaders at its best, at least in terms of presenting aliens as aliens. The rest of the shows were content to have the aliens be humans with possibly deformed little fingers. And the Gene Hackman episode went so far as to show how aliens were grown from squirming fungus-like seeds. But that's it.

See also my other post, Why I Love 'Genesis':

1) At last - even though for only a few seconds - in this episode we are privileged to see what a fully-reverted alien looks like.
"Dr. Laneer" has fully reverted to his native alien form. He is being maintained in an old-fashioned diving suit until a salinated-electrified water tank is ready to receive him. When ready, Dr. Laneer is carried by stretcher into the tank room. We see the contours of his alien form pressing against the stretcher's canvas.

Then the stretcher is slowly, carefully, opened, and Dr. Laneer gently rolls into the tank. The camera is very careful to only let us quickly glimpse this, which is one of the series' most sought-after bits of data... namely, the exact physical form of an "Invader" in its native state. The production values are very clever in that Dr. Laneer's alien body is viewed through a sort of veil of bubbles, and in a fluctuating almost strobe-like lighting effect.

WHAT is seen is hard to describe - and obviously, this is what the producers intended. Certainly it looks alien enough to make us exclaim - as the police officer does who sees it in the Prologue - "What is it... What IS it??"

Freeze-frame and image capture seem to show an extremely vague basic humanoid form, but only inasmuch as the body seems to have a head organ, a thick "abdomen", and two posterior extensions (which the electrical-salinic process will eventually expand into the legs of Laneer's human form).
It seems to be about three and a half feet to four feet long. The head organ is bi-lobed; possible bicephalacy is suggested. The rear portion is also bi-lobed but lengthened, giving a vague suggestion of stumps, thick fins, or rudimetary legs.

The genius of the scene lies in its short duration, and the indirectness of its presentation of an alien in "his" unadulterated native form. We see it, but at the same time we are tantalized and left quessing, with a sense of horror and fascination.

2) In this episode, several other alien technologies are also shown. At the Sea Lab, Dr. Laneer's research has discovered a way to generate life in a tank of liguid. Thus, this age-old dream of human alchemy has finally been achieved - but by an alien scientist. The episode does not follow up on the implications of this great leap, but instead focuses on the regeneration of Dr. Laneer. Perhaps we are to think that this brilliant discovery has been destroyed along with the fire that ravages Laneer's regeneration lab.

3) Relatively speaking, there is an extended use of alien technology in this episode.
In the Prologue, a patrol officer is subjected to the glowing hand disk applied to the back of the neck.
Then there is the alien "life created in a tank" technology. And of course there is the new information that a fully reverted alien can be maintained in a diving suit and regenerated in a salinated, electrified tank of sea water.
And there are the regeneration tubes into which Vincent and the detective are imprisoned until the power outage permits them to make their escape.
Hence, this episode is particularly rich in exhibitions of alien technology, some familiar, some completely novel.

4) The cop-detective is a veteran actor of both the large and the small screen. Hard-bitten but sympathetic, he makes a good partner for Vincent, and one might wish that they pair up again in future episodes.

... And that's why I like "Genesis" so much.