Barnacle

Physical appearance and structure
Physically, barnacles resemble little more than a large mouth full of sharp teeth and a thin, but long and very powerful muscular hydrostat combining the functions of a tongue and tentacle, which can be retracted or extended to hang at near-floor level regardless of the animal's height from the floor. Barnacles are blind, and rely on their sense of touch to catch prey. The creatures attach themselves to ceilings or the undersides of overhangs, from which they lower their sticky tongues. Although barnacles can be found in a wide array of environments, they seem to prefer damp areas. One curious point is that while many barnacles can be found infesting the Black Mesa Research Facility, few are encountered on the species' homeworld of Xen; this is likely due to their natural predators being either fewer in number or absent entirely on earth.

Barnacles are relatively weak; a few shots from a 9mm pistol will kill them. They are easily dispatched from a distance, although players who become ensnared are usually able to kill them before being consumed. The barnacles are weak against the crowbar, which in both games, kills them in a single hit. When killed, barnacles typically disgorge bones and other remains of recently-consumed victims.

In Opposing Force, a scientist refers to removing a barnacle from its "point of gestation". This suggests that a barnacle will naturally spend its entire life attached to the spot where it was born. However, it is unknown exactly how barnacles reproduce, or how the offspring spread to different environments and attach to ceilings. Real barnacles use mobile larval stages and the ocean to disperse, but no equivalent life history stage has been observed for Half-Life's barnacles (as a sidenote, the scientist's use of the term "gestation" may be incorrect depending upon how barnacles reproduce and disperse). However, it is known that they are attached very firmly and permanently, as evidenced by their continued attachment after death. In Half-Life 2 and its episodes, the Barnacle has a "second mouth" that it uses to eat and when killed, the mouth hangs out from its actual body that can be seen at all times while the original barnacle has small inner teeth inside its thick "lips".

Behavior
Barnacles are relatively simple creatures. They do not appear to possess any degree of intelligence - instead of actively hunting prey, they feed by reacting to external stimulus, in the form of animals which come into contact with the barnacles' tongue. Although the creatures do not demonstrate any intelligence in hunting, they possess an uncanny ability to attach themselves to ceilings overlooking thoroughfares frequented by potential prey, although some do seem to hide in corners far away from routes used by prey. Similarly to real barnacles, they typically occur in groups or colonies. These are found throughout the entire Half-Life series, and the player often encounters small areas populated with large numbers of barnacles, which do not seem to be in competition with each other.

Trapping and feeding abilities
When an object, living or otherwise, comes into contact with the tongue, it somehow grabs the object; the exact mechanism behind the tongue's stickiness is unknown, although Valve released a picture showing Gordon Freeman with a Barnacle's tongue wrapped around his neck and seemingly trying to strangle him. The barnacle then retracts its tongue, drawing the object towards its mouth. Upon reaching the mouth, the object is quickly evaluated by the barnacle: objects considered edible by the barnacle are crushed and consumed; inedible objects are released. Barnacles appear to be capable of killing and digesting a human being within only a few seconds, stripping the skeleton of flesh and spitting out indigestible bones. As the barnacles are never seen moving, it can be assumed that their food requirements are relatively low and that they can survive for extended periods without food, perhaps explaining their sedentary lifestyle.

In the original Half-Life, humans become immobilized when caught by barnacles - although they struggle when wrapped in the creatures' tongues, captured humans appear to be incapable of fighting against the barnacle when they reach the mouth, even though they are clearly still alive. It is possible to save a NPC from a barnacle by killing it before he or she is consumed. The NPC will then simply drop, unharmed.

In Half-Life 2, barnacles take a few moments to differentiate between edible and inedible material. Consequently, objects such as tires, barrels or pieces of wood can be used to "distract" a barnacle and allow a player to easily slip past it. In a similar fashion, by setting alight an explosive barrel at the right moment, a player can kill a barnacle attempting to eat it (or even several closely-placed barnacles). It also seems that the Barnacle tries to wrap its tongue around the target tight enough to break bones and kill it. This is based on the observation that birds as well as Resistance or Combine troops die before they even reach the main body, merely by virtue of being caught. Turning on closed captions and then being captured by the barnacle reveals the sound file is called "Barnacle Neckbreak"; also, in a Half-Life 2 concept art, a Barnacle's tongue is seen wrapped around Gordon's neck, trying to strangle him. Gordon Freeman is unaffected by the tongue's crushing attempts, probably due to his H.E.V. suit's neck-covering extension. An alternative explanation might be that tongue incapacitates the barnacle's prey by means of some poison. A third theory is that the barnacle snaps the spine of its victim, which would explain why its prey doesn't fight back and dies before consumption, and also why the "bone break" doesn't affect Freeman. It is interesting to note the manner in which a barnacle in Half-Life 2: Episode One deals with a Zombine: after capturing a Zombine, the barnacle will only eat the attached headcrab, while the beheaded body is dropped. On the other hand, barnacles have been recorded consuming the entire body of unaffected Combine soldiers. This could be considered evidence that barnacles dislike the taste of rotting or decomposed flesh, and would be consistent with their rejection of decomposed bodies thrown at them by the player. A glitch might occur where the barnacle kills a zombie of any type and throws the body to the ground. However, it will continue to pick it up and discard it until the body is moved.

In Half-Life 2: Episode 2, Barnacles are seen trapping and eating Antlions. They are also seen trapping Acid Worker Antlions, but as the Worker explodes as it is bitten into, this invariably results in the death of both parties.

Barnacle as a weapon
In the expansion pack Opposing Force, Black Mesa scientists have been able to successfully detach barnacles in good health, albeit with a high failure rate. The player can use a detached barnacle as either a weapon or as a grappling hook, However, it can only latch to organic material, such as alien fungi or moss, occasionally bearing ammunition for the slime trooper weapon as well as a handy melee tool to devour headcrabs and shockroaches. While holding the Barnacle, it squeaks and moves its fangs for an unknown reason.

Trivia

 * Barnacles can eat anything from humans to Vortigaunts to headcrabs, but if a barnacle eats a poison headcrab, it will die immediately after eating it.
 * After a barnacle eats two or three headcrabs, it dies.
 * If a barnacle is fed an Antlion Guard, it causes a glitch where the Antlion Guard is sucked through the ceiling.
 * In Garry's Mod, anything eaten by the Barnacles can be seen going up over the ceiling, probably a glitch due to the unprogrammed behavior to the victim.
 * In the Xbox 360 version of The Orange Box, it is possible to get an achievement called "Barnacle Bowling" by killing a large group of them via explosive barrels.
 * If a barnacle is fed a Rollermine, the rollermine will not move and you can walk through it. That barnacle will then not eat anything else fed to it.
 * Barnacles will even eat very large and tough enemies, such as Striders and Gunships, killing them.
 * The underside of the Barnacle weapon can be seen in Opposing force, which looks to have a star-shaped hole on it.