Flickr user modern_fred's Japanese movie monster scan collection includes a few vintage illustrations detailing the innards of Godzilla and other famous kaiju.

Anatomy of Godzilla --

This anatomical sketch of Godzilla reveals a relatively small brain, giant lungs that allow underwater breathing, leg muscles that can support 20,000 tons of body weight, and a "uranium sack" and "nuclear reaction sack" that produce radioactive fire-breath and energize the body.

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Anatomy of Jiger --
Jiger reveals her inner self

According to this anatomical drawing, Jiger has a pair of horns that can shoot missiles made of hardened saliva and one that fires a deadly magnetium (?) beam.

Anatomy of Jiger --
Jiger (uploaded by Paulkaiju)

Other characteristics include extremely powerful suction cups covering the entire body, an organ that enables Jiger to spit jets of seawater at 300 kilometers per hour, a stomach that can melt iron ore, and a tail that functions as an ovipositor.

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Anatomy of Mothra larva --
Mothra larva

This anatomical sketch of Mothra in larval form shows a robust jaw, an enormous stomach, an elongated silk-producing organ, a row of breathing orifices on either side of the body, countless cilia on the bottom surface, and a rudimentary nervous system consisting of a cerebral ganglion and a network of nerve ganglia distributed across the body.

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Anatomy of Guiron --
Guiron -- a peek inside the belly of the beast

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Anatomy of Anguirus --

This anatomical diagram of Anguirus shows eyes that can detect infrared light, a pair of sub-brains that control the forelegs and rear legs, highly developed rear leg muscles, and a heavily spiked rear carapace.

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This illustration of "Flaming Monster Gamera" (from An Anatomical Guide to Monsters) reveals eyes that can see in the dark, arms strong enough to lift and throw a 50-ton boat, and "fire sacks" that let Gamera shoot flames from his hands.

Anatomy of Gamera --

The illustration also shows a series of sack-like organs for storing lava, oil, coal and uranium (like Godzilla), as well as balloon-like organs in the legs that can blast air through the bottoms of the feet.

[Link: modern_fred's kaiju eiga photoset]

Related: Kaiju art collection