Long One

The Long One is a mysterious parasitic alien and the only one of its kind. It travels across the universe, in search of life forms to kill and feast upon and is hellbent on consuming and destroying everything on a planet until nothing is left. The Long One exists as many parasitic forms, working like a "conscious disease". The Long One arrived on Earth via a meteor, that landed in the fictional town of Wheelsy, in the 2006 film Slither.

Origins
It is unknown where the Long One originated from or how long it has lived in the universe. No details are given about its habitat except that it came from outer space. It came to Earth on a meteor, but because not much is known of its homeworld conditions, the nature of its arrival is speculative as well.

Physical Appearance
The Long One exists in many physical forms. Though it isn't explicitly mentioned, it can be categorized into four entities or forms, (at least, for the purposes of this discussion). These are the Needle, Primary Host, Womb and Slugs (see notes below).

Needle
The Needle is the original form of the Long One. It is called Needle, because of its resemblance to a needle, albeit much thicker and seemingly hairier. It is encased in a small gelatinous sac, which open up when the Long One is ready to shoot out and implant itself into a host. Once implanted into a host, the Needle merges with the host and turns into the Primary Host.

Primary Host
The Primary Host is a changing and growing form of the Long One. It starts out looking like its original host and is continually deformed eventually resemble a giant squid. It is characterized by its whip-like tentacles emerging from various points of the Primary Host's body. When the Needle implanted itself into Grant Grant, his physical appearance could be compared to a cross between a squid and John Carpenter's rendition of the Thing.

Womb
The Womb is another form of the Long One, which is created by the Primary Host by implanting part of itself into another host. The Womb continually feeds and thus continually grows, still resembling its host, but becomes much more massive to the point of becoming a giant sphere. At a certain point, the Womb cannot grow anymore, and its body rips itself apart releasing the Slugs.

Slugs
The Slugs are small red worm-like forms of the Long One, which move by slithering or swimming. Like the Needle, the Slugs also implant themselves into hosts, although in their case, they are swallowed; changes are mostly behavioral and not physical. A physical change that is noticeable is the bleeding from the mouth, due to the forced entry of the Slug.

Biology and Physiology
The Long One's physiology is not explained, though it is evident it has a complicated life cycle and physiology, capable of space travel and transforming into various physical forms. What is known is that it is a parasitic creature, capable of controlling hundreds, (possibly even more) hosts at any given time through its various forms. The Long One can control any life form, (not just humans), but may prefer humans due to their extensive cognitive and physical abilities.

Needle
The Needle is encased in a small gelatinous sac, probably as a defense mechanism for when it lands on a new planet like Earth, whose atmosphere is foreign to it. It can also be thought as a protective shield for the small fragile Needle form of the Long One. The sac opens up when a viable host comes nearby and the Needle shoots into its abdomen and travels to the brain. In the case of Grant Grant, the Needle shoots itself into Grant's breastbone, slithers up his brain stem and settles in the hindbrain region, initially taking control of his cerebellum and eventually (as suggested, though not mentioned) moves up to control his cerebrum. The cerebrum and cerebellum function together to control all voluntary muscles as well as aid in sensory, language and memory processing. This control becomes evident when it is observed that the Long One not only control Grant's movements, but also shares in his memories and communication skills. It is also evident in how the Long One, across all its forms, functions on a "hive mind" mentality, which is why the Womb, the Slugs and all infected hosts function as a singular unit.

Primary Host
The Primary Host therefore acts sort of like a home base for the Long One, allowing it move and speak, something that its original form as a Needle couldn't do. The Primary Host will start to have rashes, eventually growing tentacles and evolves into a grotesque monster. Its tentacles are strong and flexible enough to kill animals (and presumably similar life forms throughout the universe), which is important for the Long One as it feeds on raw meat. It is also strong enough to vertically split a human being. The Primary Hosts moves by slithering, despite its larger size; however it stops moving at some point, when it is no longer threatened or when it is too large due to merging with more hosts. The Primary Host does not lose total control however. This is evident in how the Long One still is longing for Starla, Grant's wife, which is indicative that Grant is still present in the decision-making process. It becomes more evident in how Grant is able to suppress the Long One from killing Starla.

Womb
The Long One also needs a Womb to which can give birth to Slugs. The Primary Host impregnates another host by injecting its material through two tentacles emerging from its body. In Grant's case, two tentacles extending from a hole in his abdomen act as the vehicle to transport the Slug eggs into Brenda's stomach. The impregnation requires two tentacles to penetrate the host's body, as one is not enough. The Womb can still think and speak with some semblance of rationality, but is distracted by an overwhelming desire to eat (signifying a control of the hosts hypothalamus), busying its time consuming raw meat. The eating allows for the birth of hundreds, maybe thousands of Slugs, which bursts out ripping the Womb apart.

Slugs
The Slugs can be thought of as the soldiers or extensions of the Long One, capable of infecting more hosts at once. Unlike the Needle though, the Slug's control on a host is limited only to movement and communication. No cognitive or independent ability is ever manifested. Because of the hive mind nature of the Long One, Slug-controlled hosts have the same memories and exhibit the physical mannerisms as the Primary Host. Slug-controlled hosts can also spit an acid-like substance, that causes a burning sensation and increased blood pressure of the affected area. The Slug-controlled hosts live for two purposes: To gather food for the new Wombs, and to synthesize or merge with the Primary Host, allowing the Long One to become all the more gigantic and thus consume more.

It is possible to remove the Slug before it manages to completely enter the host. This has only happened once with Kylie who managed to fight the slug that was trying to enter her long enough for it to weaken and allow her to pull it out of her mouth. This encounter also transferred some of the hive memory to Kylie allowing her knowledge of what was happening in the town.

Parasitic Cycle
The Long One's goal of total destruction and consumption cannot be achieved through one physical form, especially not in the form of a small Needle; this means that as terrifying as the Primary Host is, it cannot kill every single individual without threat of annihilation. In order to control more people, it needs to spread parts of itself, in the form of Slugs, which can infect and control other hosts; the Slugs are born from the Wombs, which the Primary Host and eventually the Slug-controlled hosts feed.

The Long One's complicated life cycle can be summarized in these six events:
 * 1) The Needle (Original form of the Long One), shoots itself into a host, and proceeds to systematically control it, becoming the Primary Host.
 * 2) The Primary Host impregnates another host to act as the Womb.
 * 3) The Womb is continually fed raw meat. It is implied that the consumption allows for the synthesis of the Slugs, which bursts out when the body of the Womb can no longer grow. The Primary Host can create as many Wombs as it needs, though initially focuses on one as it is hard to feed.
 * 4) The Slugs find other hosts to control, entering via the mouth and settling in the hindbrain, controlling basic movement.
 * 5) When the Primary Host is no longer threatened, the Slug-controlled hosts merge with the Long One, creating an even more massive creature. Slug-controlled hosts also help in collecting food to feed the Wombs.
 * 6) The Long One leaves as its original Needle form when everything has been consumed. Its method of moving to another planet is not mentioned though.

Death
The Long One is killed by the efforts of Police Chief Bill Pardy and Starla Grant. In the Long One's attempt to impregnate Bill Pardy (making a Womb out of him), Bill manages to stick one tentacle into a propane tank, sending the higher pressure gas into the body of the Primary Host. Starla then shoots the Primary Host, igniting it and causing it to explode.

Upon the death of the Long One, every infected life form is killed, including the Primary Host, the Wombs, the Slugs and the Slug-controlled hosts, due to the extensive damage of the parasite.

Trivia

 * A vague vision from when Kylie Strutemyer almost ingested a slug shows another planet in which the Long One kills a dog-like creature, eventually controlling some and physically merging into one organism. There is no mention of this planet being the Long One's homeworld, another alien world or even Earth. It does however indicate that the Long One has done this killing spree before.
 * It isn't mentioned whether the hosts die upon the death of the Long One, though they can be considered clinically dead due to the extensive brain injury.
 * In the film Guardians of the Galaxy, Slugs can be seen in one of the glass containment cells in The Collector's Museum.