Trilobite
The Trilobite was a massive, octopus-like creature resulting from Charlie Holloway being infected with Chemical A0-3959X.91 – 15 and engaging in sexual intercourse with Elizabeth Shaw. Similar to a Facehugger, its primary purpose appears to be to implant an embryonic Deacon into a host. It is not made clear if the creature was conceived by their intercourse or if the infant creature merely moved into Shaw from the infected Holloway. It is unknown how the creature gets out of its host "naturally", as Shaw removes it prematurely; however its exaggerated movements prior to this suggest that it "bursts" like Bellybursters, emerging from the host's abdomen.
