The Barnacle is a type of Xen creature that has so far appeared in all of the Half-Life games. Appearing as motionless sacks of flesh, Barnacles can be found hanging from ceilings with their long, rope-like tongues hanging from their mouths. The tongue secretes an adhesive substance that will stick to anything and can support large amounts of weight.
The Barnacle does not actively hunt prey and merely hangs in a single spot, waiting for prey to happen by. Any objects, organic or inorganic, that get caught on the Barnacle's tongue get pulled up into the creature's maw. Organic material is crunched up and consumed, but inorganic materials like stone and metal will be discarded once the Barnacle fails to ingest it. A single bite of the Barnacle's maw can sever a human's head from its body.
Barnacles tend to hang in dark areas where prey are unlikely to spot them. Since these creatures are immobile, they can be easy to avoid depending on the surrounding environment. They are easy to kill, but shooting them can be a drain on ammunition, especially if there are many of them in a single area. They can be avoided altogether by throwing useless objects at their tongues and when the object is lifted up, one can sneak past the Barnacle while it is chewing.