"Green Patches" by Isaac Asimov. Also published as "Misbegotten Missionary."
There's a planet where all life is part of one consciousness. All forms of life have two green patches, in the animals the green patches are where the eyes would be.
The story is about a ship that lands on the planet to do some research. Another ship had landed there once, but the crew destroyed it when test animals (and maybe a baby) were born with green patches, indicating that the planet had assimilated them.
The life on the planet wants to "rescue" humans from their individual existence - the humans, of course, have no intention to become part of the planet's communal life.
The one worm like animal manages to sneak into the ship, and disguises itself as part of a wire. The wire is only needed when the lock mechanism is activated.
The worm thing hides during the whole trip back to Earth, with the intent to "rescue" all of Earth rather than just the few on the ship.
The ship lands, the lock is opened, critter destroyed, Earth stays free.