From Black Mesa: Wiki

The Barnacle from Episode 2, also featured in Black Mesa.

Overview

The Barnacle is a stationary Xenian lifeform, encountered throughout the Black Mesa Research Facility after the Resonance Cascade incident. It has a very simple appearance, made up of a couple of components, a brownish-red outer skin layer, several protruding teeth on the outside, several inner teeth, and a long tounge, a sticky tongue/tentacle-like appendage that hoists up preys that are to be eaten.

Behaviour

Barnacles attach themselves to ceilings, in any areas, but damp areas preferably in particular. Barnacles have a very simple behaviour, that is, to simply wait for the prey to be under them, and as they get stuck, are hoisted up and eaten. Because Barnacles are blind, they rely on their sense of touch in order to find and consume prey. There is a auditory cue as to when Barnacles catch something on their tounge.

Mechanics

Bodies that hit the tongue instantly ragdoll, because the tongue is designed to work with physics objects not animated NPCs (which is why boxes and barrels and things can be dragged up). It is way beyond the scope of this mod to change that behaviour, so the Barnacles in Black Mesa will be consistent within all Source games [1].

Tactics

Barnacles are relatively weak in terms of endurance, a simple crowbar hit or several 9mm shots can kill them easily.

Media

Many barnacles camouflaged in the ceiling (chapter Unforeseen Consequences).

Barnacles' exposed tongues near the rails (as seen in On a Rail).