The mottled brown undulated along the forest floor, crawling toward the new host that had been offered to it. It wasn't fast, but the human was paralyzed, just standing there, facing away. They had been very lucky with this planet: What were the odds that they'd find a habitable world with a dominant clade with dorsal nervous systems? The humans were especially advantageous: Aside from grasping appendages, they had already built an infrastructure that was useful to them - and thus to anyone that they hosted.
The extraterrestrial alien buried itself under its new host's skin. This one would do just fine.