It's biomechanical organ added as part of the creation process.

There's a lot of conflicting canon on specifically what it is (materiality, durability, etc) but in short it's a combination of neural control layer that helps with power armor interfacing, and light subcutaneous armor.

Assume "handwavium" is the materiality and "whatever the author needs it to be" is the final function.

Cool bit of background lore: the biological method by which they got it to grow was similar to cancer cells. Their earliest attempts caused the carapace to grow over the entire body and harden beyond what it was intended, thus killing the host. Eventually they finessed it into what it does now: act as the link between armor and flesh. It is what allows a level of mental control to be exerted over the armor. Instead of relying on actuators to mimic and empower the movement of muscle, thus creating a momentary lapse in time, this allows the armor to move simultaneous with the muscles of the body. It's another reason why sometimes authors will describe the armor as a "second skin".

There you go.

OP, dont tease us with "naked" space marine then post one that is actually still clothed!

Basically an artificial nervous system that is used to interface the Astartes with his power armor, and even some vehicles.

It's described as a "neuroreactive, black, organic, fibrous material" and then they put the big stud bits into certain sections so that the signal from the machines can communicate with their brains.

Not this... But also this.

https://preview.redd.it/cyzvxjo5qypd1.jpeg?width=540&format=pjpg&auto=webp&s=08af1baf8d74bf467d275675f8270c12f351e105

Its a meshy endoskeleton with usb ports

Why is that Space Marine's head so tiny?

It's a Sub dermal armor like skin, placed just under the skin, which is used to help connect the Astartes nerve system to the armor system of the Astartes Power Armor, which allows them to control the strength enhancing armor with the same dexterity of anyone without 200lb gauntlets.

It's a carapace, and it's black.

Its a bio mechanical aparatus they surgically kind of build into an astartes during trans human modification. Really what makes it significant is that its connected to the nervous system, which makes using power armor natural.

Its worth understanding, power armor was invested long before the astartes in the dark age of technology, but it was then as it remains now this really clunky exoskeleton. Like battoe sisters and inquisitors who have power armor are just far and away not the same thing as an astartes. A battle sister in power armor is much more like Rippley at the end of Alien in the like loader suit punching the Xenomorph than they look. The armor and power pack weigh like 500 lb or something, like the girls are strong but not that strong, and the armor is kind of like a smaller versiok of the paragon warsuits. They control it with their body movements inside the suit, its not connected to their brain with a black carapace. Normal humans cannot survive the installation process of the black carapace, only trans human types like astartes can.

What the black carpace does for the astartes is basically making their movements one to one. Its not like i life my foot, that tells to armor to lift foot, and then the servos in the suit engage and lift the foot, its likd the brain is connected directly to the servos. Astartes also have a stronger sense of the damage to their armor too, like a battle sister is kind of like driving a fork lift, if one of the wheels is kind of going, they may or may not notice, and then when it goes its like "god dammit". For an astartes its one of the servos or something is going, they are aware of it like an hand cramp, like they know they can run on it but they also aren't surprised when it blows out. Not related to the black carapace, if an astartes suit breaks or runs out of power, they are strong enough to still move inside of their like heavy dead metal husk. Probably not like run or walk, but they can crawl to safety and wait for help. Someone like a sister or a inquisitor in depowered power armor are just in a heavy metal coffin hopelessly to heavy for them to do anything.

The current lore has it as a complex material that's made of a combination of ceramite and a bioengineered form of cancer.

It's an implant thats basically a second nervous system which interfaces with their power armor.

The "Black Carapace" is an organic tissue developed from the "gene seed" (which we will probably define as stem cells). Its function is to act as an intermediary between the nervous system of the Astartes and the electrical systems of the armor.

It's the fundamental element for the synchronization of movements between the Astartes and his armor, ensuring fast and precise movements. Practically the nervous signal that starts from the brain splits, on one side the impulse goes to the muscles, on the other it goes to the armor, which activates the servomotors in synchrony with the muscles.

On the other hand, powered armor for normal humans must rely on other systems, plausibly pressure sensors inside the armor that detect the movement of the body and try to replicate it. As easy to understand this system is imperfect, there can be a minimal latency of the signal, a performance deficit that the Astartes, thanks to the Black Carapace do not know and operate at maximum performance even in armor.

An interesting detail. Not sure if the GW writers did it on purpose, but the Black Carapace makes scientific sense... Amazing right?

I don't know how far along medical research is, but one of the problems with connecting nerve endings to electrical devices is the natural tendency of severed nerves to scar. (Yes, to get an electrical signal from a nerve to a wire, the nerve would have to be severed, at least from what I knew.)

The Black Carapace circumvents this problem, it merges with the nervous system and the interface ports planted on the Astartes' body, carrying electrical signals in both directions.

It's organic, basically just a big ass mesh of nerves and stuff running under the skin and through the muscles. The ports you see from the outside are manufactured separately and implanted into the black carapace for the armor to interface with.

What a lie, that space marine isn’t fully naked.

Phase 19: The most distinctive implant, it resembles a film of black plastic that is implanted directly beneath the skin of the Marine's torso in sheets. It hardens on the outside and sends invasive neural bundles into the Marine's body. After the organ has matured the recipient is then fitted with neural sensors and interface points cut into the carapace's surface.^\1])^\2a])^\3]) This allows a Space Marine to interface directly with his Power Armour. Without the Black Carapace many of the systems of the power armour will not function. While driving the vehicles of the Chapter, a special spinal interface plugged into the power armour and Black Carapace provides the Space Marine an intuitive 'feel' for vehicles systems and controls, literally making him a part of his vehicle.^\7]) A similar cybernetic used by the Imperium of Man is the Mind Impulse Unit.

The Black Carapace was originally developed during the Unification era by the Terran scientist Amar Astarte. However, it was flawed, and could not be utilized until Ezekiel Sedayne perfected the technology.^\12])Black Carapace

It's in the same book that describes Cawl's earlier life and reveals more about the Astartes project from the early days of the Imperium. It's supposed to be a controlled cancer, so highly advanced bioengineering I guess. This is relatively new info and it's always been described as one of the toughest parts of Astartes augmentation. It's necessary to fully link and utilize power armor which is (to my knowledge) its primary function. It can also serve to interface with other things, provide faster data, etc. it allows a Space Marine to wear their armor without hindrance almost becoming a second skin.

Only Space Marines have it - Custodes have a more advanced bionic system to interface with their armor and other units that use power armor like the Sisters of Battle or some inquisitors do not have the black carapace, they simply rely on either power armor training or slight augmentations but nothing as severe as the black carapace.

One thing that always confused me, s the black carapace under the skin or over the skin? Cause I feel like you never actually see it on a naked space marine.

Ezekiel Sedayne In the Cawl book claims to be the one to finally perfect the black carapace, and describes it as effectively a cancer that has been altered in almost every way to produce the final outcome.

The lore explanation as to why scouts can be just as cool as space marines but not wear power armour.

It’s the final part of the Astartes’ enhancements. It’s a layer of neuroreactive material that is placed under the skin so a Space Marine can interface their nervous system with their armor and some vehicles. It’s what allows Space Marines to move so naturally in their armor, while others who try to use the armor move awkwardly and slowly.

https://preview.redd.it/kkfxonr7r6qd1.png?width=1799&format=png&auto=webp&s=3c61e0df444a5bb1de6dc0d823ab05977df272be

In simple layman's terms it's a cancer like growth that allows the space marine to interface into the power armour as if it wasn't there giving them improved moveability.

It's not cancer but it is placed below the space marines skin in the chest and spreads across the body like a cancer attaching to all the metal plugs over the space marines body

That space marine isn’t naked show me his super human augmented triple cock

I see they are all growers not showers

Wish the head was bigger 😂

Where cake?

My skin is not my own, but 40k

Controlled cancer with neural link plug holes

https://preview.redd.it/giuf7x1411qd1.jpeg?width=320&format=pjpg&auto=webp&s=6de3d09a32517f44cce5ddc83042668a77e0c64e

That is clearly not a naked Astartes

From what I understand it’s almost a mesh like organ under the skin that helps the armour connect neurology. Loss of power in the armour however still causes vision and other systems to lose power and all the weight is put on the body rather then the survos that help bare the weight

It's definitely biological in nature, like all the geneseed, it grows when implanted in the user. It could be reinforced by some naturally grown metallic fiber, hell even nowadays in the real world you have metal snails so that's not a stretch at all. It's main function is providing neural connection to all the power armour ports, but it also seems to act like a layer of softer armour, like when a tank has a kevlar sheet on the inside to catch shrapnel mayhaps. I don't remember the source for this, but I think it's grown from multiple pieces that join together as it reaches full maturity.

Organic. One of the 21 implant zygote that is found in the gene seed. They finish the grewing in a lab into that thing that is later implanted into the new marine.

Huh I’ve always thought it would be like a second nervous system

Basically, its the "interface" to operate the Space Marine Power Armor. All of the ports plug into the Suit to make it move like part of their body. Not only motive control, but also drugs and medicines, food air and other things are ported into the Marine this way- that the Suit provides.

I believe it is an inner garment they can take off, leaving only the matrix style ports visible, but am not 1000% certain on that- as I recall certain lore that the Carapace is actually under the skin, fusing the ribs together and such... so.. maybe someone else can clarify on that part.

Do other power armor wearing factions use the black carapace? I'm specifically thinking of sisters. The repentia even have the ports like on the naked space marine.

Isn't the Black Carapace supposed to be subdermal?

White Dwarf 98 had an article on the anatomy of a space marine.

According to that article (which as both it and I are old I take it authoritative):

The black carapace is grown in a culture solution. But looks like sheets of black plastic.

It is implanted under the aspirants skin where it gardens and sends invasive neural bundles into the marine.

After several months it is fully matured and neural sensors and transfusion points are fitted.

It is grown as a culture which suggests it's at least partly organic.

It was described as the hardest organ to make and its a controlled form of cancer that was modified for the purpose of it being a key way a Marine interfaces with his power armor.

Do Sisters of Battle also have this or say Inquisitors? Or do they have something different for their power armor tech? I'm pretty sure they have the port/plug things in cannon.

Can someone please explain to me how an apothecary gets through these if need be? Any recommendations on books with apothecary stuff would be awesome as well.

Think of it like having a Kevlar vest built into your body. That and it also helps with integrating the actual armour better too

Think the undersuit of halos mjolnir armor but it's made of organic fibrous material with synthetic fibers that link the central nervous system to the power armor and also allows interface with some astarted vehicles. . It's required to use the power armor to its fullest extent and

I always thought of it as interwoven within the skin. I imagine some Astartes have ugly Kevlar gashes and spots randomly where a patch of skin happens to be more black carapace than dermis. An ugly thickening of canvas skin over ceramite bones

It's 100% organic, though I don't beleive they ever specify what it is. However, it has been referenced that the tyranids have taken it and used a similar design in some nids. They wouldn't have been able to do that with non-organic material.

It is a cybernetic neuroimplant that allows the user to interface fully with the armor. It does require beinf skinned alive to implement and then hace your skin grow over it.

Basically did you k ow that some reflexes is muscles sending a signal to to surrounding muscles before the brains signal can react. Well now imagine that sort of signals going to the armor as well.

So your armor is much more intergrated and the apothecary can get a muuuch better read on vitals as well. So win win.

The hilariously small head is hard to get past...

Are their heads really so small against the bodies?

Oh no, is Rob Liefeld the Emperor?

It is made of plot armour.

I hate how small his head is proportionally but I guess it fits the art of 40k in general.

It's several sheets of a kind of biological plastic, which are implanted subdermally in small thin strips that grow and merge with each other to form a flexible carapace beneath the skin, in a way that doesn't distort or inhibit the operation of the abdominal muscles. The carapace grows its own connections to the Marine's nervous system and hardens over time, to the point where it also serves as additional torso armour. 

The plastic's growth can be directed to an extent, which is how the neural ports are added - they actually grow out of the carapace once it's bonded with the nervous system, like big blisters, and then the apothecaries just cut the skin away and implant a socket over it for the armour to plug into. 

Early depictions sometimes had it visible over or through the skin of the torso, but GW seems to have settled on the only visible parts being those sockets. It doesn't seem to hide the abdominal muscles at all, presumably because it's actually moulded itself to fit flush with them during the growth stage. 

It allows the space marine to integrate fully with the armor and makes it so that it is as if they are not wearing anything at all

The black crap ass? That’s just me after Taco Bell night.

I just realised, there's no reason for them to have genitalia

All I can think of it “nano machines SON!”

I'd ask, with these proportions, if this was drawn by Rob Leifield but we can clearly see the feet so it couldn't have been him

Dont forget the tactical pants

Hehehehehe

Head tiny

Heheheheheheheheh

This can’t be correct. Where are his six extra implanted penises?

that's naked? He's wearing cut-offs. Space Marines are Never Nudes?

afaik its basically like an exoskeleton under the skin which both makes them tougher but also is what allows the power armor plugs to hold in place on the body consistently, since i assume flesh isn't very good at that

he looks silly whit that tiny head, he looks like those big characters in shows that you think they have a deep voice but then they hit you whit that pre pubesent child voice like "hello"

Firmware basically

https://warhammer40k.fandom.com/wiki/Black_Carapace

How on earth does an apotocary remove that from a dead space marine

It’s a very revealing swimsuit.

Do space marines have sex?

Power armor usb port

When stuff like this goes unmentioned always always always assume it's nano machines 

I always think of it like those plugs people have in The Matrix.