-1 SECTION 2:
INTERNAL IRKEN PHYSIOLOGY
ORGANS
Predicting the types of organs and organ systems an alien race would possess would be highly speculative and nearly impossible. Fortunately, however, we know that Irkens have two main organs, which are the brain, and the squeedledyspooch. We also know that Irkens have blood. Using these axioms, I have been able to determine to a reasonable degree how the internal functions of an Irken's body is carried out.
The brain of an Irken is comparable in complexity, function, and makeup to that of a human. As in all animals, the Irken brain is located in the skull, inside a braincase, and is responsible for carrying out not only thought, but also breathing, sight, and all other voluntary and non-voluntary responses in the Irken body. Irkens, however, seem to be able to exist with their brain removed, due to a biomechanical "PAK", which is connected (probably) to the brainstem via nerve-like conduits. The PAK acts as a life support mechanism, as well as a backup database for the individual Irken's memory and personality. Unfortunately, Irkens are highly dependant upon their PAKs, and the removal of one results in the death of that Irken after a period of about ten minutes, my guess from full cardio-pulmonary arrest, and the cessation of all body functions, including probably lastly, brainwaves.
The internal organs of the Irken have evolved into one large organ called the squeedledyspooch. Logically then, the squedledyspooch is responsible for respiration, digestion, circulation, hormone production and waste removal, just to cover the basics. But how can one organ do all of this? Simply put, it cannot. A heart cannot digest nutrients, just as a kidney cannot control blood flow to the body. Though the squeedledyspooch is classified as one organ, it is actually very compartmentalized, with each section performing a specific task In a system such as this, major arteries and tracks of piping designed in our bodies to carry blood, waste, and food between organs and organ systems would be negated, making for actually a simpler, more efficient organism overall.
The nervous system, like the brain, is on par with that of humans. Irkens feel pain, heat and cold in the same manner we do, although they seem to have higher tolerances to heat and cold, probably due to the environment of planet Irk.
Not much can be said about Irken blood or circulation, except that Irkens are warm-blooded creatures. Perhaps it is by their natural design, or maybe their PAKs allow them to be endothermic, but biologically, Irkens would almost HAVE to be warm blooded due to their heat and cold tolerances and their quick, nimble nature. This however, is speculation, and there just isn't enough empirical evidence to back up anyone's theories about the circulatory system of Irkens.
DIGESTION
As we have observed, Irkens have an affinity for snacks, and Zim can eat waffles with no ill effects. Therefore, we can assume that Irkens have an ability to digest food nearly the same way we do. Zim, or no other Irken, for that matter, however has been seen either using or making reference to using a lavatory. As for the question of removal of waste from the body, barring the theory that Irkens do normally secrete waste, but this act simply hasn't been mentioned on the show, I have two possible explanations:
1) It is possible that the Irken body maintains such a high metabolic rate, that 100 of food taken into the body is entirely assimilated, and no waste matter is produced. Though rare, this does occur in the animal kingdom, and does explain Zim's "allergy" to certain processed foods and meats, which could not be digested by a simpler digestive tract, and would, in theory, act as a toxin to the system. In fact, even in humans, vegetable matter is more readily digested by the body, while meat actually "rots" for a while in the digestive tract while the weak acids of our stomach and intestines attempt to break down the tough substances that comprise a delicious porterhouse steak that our system simply wasn't designed for.
2) A plausible theory, yet one I have yet to hear in a fan fiction (clears throat), is that Irkens possibly excrete waste through their skin. Once again, this would account for Zim's inability to digest certain foods. Simpler, grain-based foods (ae, waffles and chips) could be reduced to such a final state during digestion, that what little waste by-products could be osmosed through the dermal layers as a liquid waste.
As stated before, some foods act as a bio-toxin, even as caustic to the body of Irkens. I have explained how some foods could be dangerous to an Irken's system, but what about others? Simple. Irken tissues react violently when exposed to water. So it is a given that foods with moderate water content would act the same as pure water to an Irken. Ice cream, fresh vegetables, and tender steak sounds like a delight to us, but to an Irken, would be the equivalent of a human dining on lye powder and drinking a nice cool glass of nitric acid.
As to why Irkens are "allergic" to water, that will be explained later.