Parasite (Parasyte)

Parasites are a race of extraterrestrial beings that have a desire to take over a human host. They came from small eggs which floated down to Earth from unknown origins. They start out as a small worm that enters the body through some kind of orifice (i/e: Ear, nose, mouth). If the host resists, the parasyte has a sharp spine which allows it to burrow directly under a human's skin. Then, they travel to the head and take it over, controlling the entire body with the central nervous system. Sometimes, they will accidentally take over a different part of the host, where it forms a symbiosis with the host, while both beings retain separate intellects. These parasites are usually regarded as either pathetic failures, or dangerous threats.

Anatomy & Physiology
Parasites begin their lives as small worm-like creatures, with "heads" tipped with a large, drill-like spike. In this state, the parasite seeks out a host to merge with, burrowing into the brain if possible. Once this happens, the parasite's cells meld seamlessly with those of the host. This process is irreversible, and if it affects the host's head, the host's consciousness is effectively destroyed.

From the neck down, a parasite's host body is unchanged. However, the head is drastically altered, consisting of parasite cells that serve both as muscle cells and as neurons. The parasite-head normally looks exactly as it does before the parasite infested it, but it has a nearly unlimited ability to change shape. A parasite can change its facial features to look like another person, or it can attack by shaping its head into bladed tentacles or an enormous fanged maw. While the body does not have these abilities and cannot regenerate, the parasite can force it to continue functioning even in the face of damage that would kill or cripple a normal human. Generally, only destroying the heart is sufficient to kill it quickly.

Parasites infesting other parts of the body or other species also exist. In the former case, it does not control the host, but is able to manipulate the body part it has merged with. In the latter, the parasite is fully sentient and capable of speech, even if the host is not (e.g. a dog).

Diet
Parasites are carnivores. Specifically, they are instinctively driven to hunt and kill members of their host body's species, using the host as a disguise. Therefore, human parasites mostly feed on humans, while a parasite infesting a dog will hunt dogs.

Reproduction & Life Cycle
How parasites reproduce is not known, and even parasites themselves do not know where their spores originate. They do not have memories of their time as spores or worms, and their sentient existence begins when they infest a host. Additionally, since parasites do not alter the host's reproductive organs, two parasites mating simply produces a normal human.