-1 SECTION 1:
EXTERNAL IRKEN PHISIOLOGY
We all know from the show the basic physical profile of a typical Irken: short by our standards, large head, green skin, a pair of antennae, three-digit hands including opposable thumbs, and large, pupiless eyes which can be red, blue, purple, pink, or several other exotic colors. So, let's start off with the basic question: What classification of organism to place an Irken in? In order to answer this, let's think about what we know about the race.
HEIGHT
According to the show, the typical Irken stands at approximately the same height as a human child of about twelve years old. This means that to us, they should be an average of four, to four and a half feet in height, and proportionate in their weight to that of an equally tall human of average build. There are exceptions to this rule, though. The show has depicted some Irkens as being slightly taller than the norm for their species. These, due to a height-based hierarchy, usually hold positions similar to what we would consider military officers, and are often seen on the bridge of the Irken armada's flagship, The Massive, performing duties as deck officers and communications specialists.
As stated before, the Irken race is a height-based hierarchy, with the tallest of the race dubbed simply, albeit aptly as "the Tallest". Unlike in most cultures and governments, the Tallest are not elected, nor must they possess any superior knowledge, strength, or leadership qualifications. It seems height is the only factor governing becoming the leader(s). The title of my Tallest is in fact, our equivalent to "my lord" or "my king". The Tallest depicted in the show, Tallest Red, and Tallest Purple, seem to be wearing a sort of biosuit, begging the question of whether or not the Tallest are truly as tall in their original form.
To get back to the physiology department, however, despite anomalies in height, most Irkens are actually quite small. As impertinent as this may sound, it is actually an important bit of information when determining such things as environment and internal anatomy.
The short stature of most Irkens can possibly mean that the gravity on Irk is greater than it is on Earth. Gravity pulls down at a constant rate, slowing the upward growth of things. If gravity had a little less pull on Earth, we as humans could possibly grow to ten feet in height. Ergo, we can say that planet Irk must possess a gravity field slightly greater than Earth's, meaning that it is probably larger in size.
Perhaps this is not the case, though. Perhaps the Irkens have naturally evolved a short stature, much like higher primates such as orangutans and chimpanzees. Biologically, smaller means less food intake, faster growth rate (usually), and more mobility. It also means a higher population is capable of living in a smaller area without depleting natural resources and overcrowding, a necessity for a race such as the Irkens who seem to seek to invade and colonize every habitable world they find.
ANTENNAE
Antennae are a good place to start when attempting to classify an organism. Note that they are long, black, and seemingly hairless, defeating the theories that they are akin to hair or horn. The antennae are movable, as we've seen in The Nightmare Begins. This indicates the antennae are probably joined to the head (and not the skull) by a series of fibrous muscles at their base, which are used to move the antennae, much like the muscles attached to the human eye. The antennae are undoubtedly segmented, as nearly all such appendages in the natural world are. A species doesn't just HAVE antennae, however, and they probably serve as a tactile organ, much like the "feelers" on terrestrial arthropod insectoids. There is no doubt that antennae of that size would be chock full of specialized receptor nerves, and would allow for the detection of minute disruptions in the surrounding air, allowing an Irken to detect another presence or movement even in absolute darkness, making them quite efficient hunters.
As seen in a few scenes, when under stress or in a state of fear, an Irken's reaction is to lay the antennae back, against the head. It can be assumed that this is an evolved hereditary response, designed to protect the appendages in the event of danger. It can be assumed that when alert, the antennae would stand erect, much like the ears of members of the felidae and canis families. The lack of visible ears denotes that the antennae probably serve an auditory function as well, picking up vibrations in the air from sounds, which are carried via specialized nerves to organs inside the skull, where the vibrations are interpreted into dialogue or actual audio signals by the brain. In this sense, the antennae serve basically the same function as the human ear, possibly with comparable cochlea and eardrum-like organs attached in the same manner. Because of the sensitivity of antennae over ears, it can be assumed that like insects, Irkens can detect or even hear a wider range of sounds either too high or too low in frequency for human ears to detect.
EYES
Irkens possess large eyes that in males of the specie seem to be more rounded or ovular, while in females, tending to be more horizontally elongated and angular. This possibly aids in the identification of sex among Irkens, as physiologically, males and females appear to be quite similar in most other aspects.
The most common eye color seems to be red, or shades thereof, even fading into a pastel pink shade. Purple appears to be the next most common eye color, once again varying in hues. Blue eyes can also be noted among some Irkens, though it seems to be more rare, and other colors probably exist, albeit in very rare cases, and may be examples of genetic mutation.
The size and colors of the Irken eye can be taken as indicative of the kind of vision that Irkens possess. Red, blue, purple, these are "colors" of the electromagnetic spectrum, and at that, colors such as infrared and ultraviolet, which the human eye cannot see. Perhaps The Irkens have evolved eyes, which not only detect the "invisible spectrum", but also utilize it for their advantage, being able to see heat generated by a living body or machinery, as well as ultraviolet radiation.
To evolve such a function, we can deduce that Irk is a planet subjected to far more intense radioactive bombardment from space. If the star Irk orbits is slightly closer to Irk than our sun is from us, or it is a binary star, or even a more massive star, this evolutionary path can be accounted for. Adversely, this trait can also be the result of Irk being subject to less light and solar radiation than Earth, and be evolved as a way for Irkens to have excellent vision on a planet darker than ours. The latter theory would also explain why Irkens are equipped with a far better sense of hearing than humans.
Having no pupils, Irkens probably are incapable of focusing on a particular object, but probably have a far greater panoramic view than we do. The relative size of the eyes would only increase this field of view, perhaps to a totality of 180 degrees on the horizontal plane. Once again, this would be a boon to a warrior race such as the Irkens, allowing them not only a much greater field of view in battle, but possibly allowing them to see the heat signatures given off by the enemy, much like the sensory organ of the pit vipers, and the genus crotalus.
SKIN
Irken skin in a uniform shade of green. Given the fact that Zim seems to be allergic to human food, this has led to the conception of the belief that Irkens are capable of some advanced type of photosynthesis. I personally have my doubts concerning this belief for a number of reasons.
Photosynthesis relies on specialized cells, as found in the leaves of plants, which act as photoreceptors, basically miniature solar panels. Using energy from the sun, along with other necessary chemicals (ae. chlorophyll, H2O, CO2, etc.), the plants are able to synthesize the energy needed to grow and reproduce. Although photosynthesis could exist in a modified form, using other compounds or chemicals, such as hydrogen, ammonia, or methane combined with solar radiation, I still find this idea rather implausible.
Think of a tree. A tree doesn't need to move, to walk, talk or run, and it has very simple biological functions. And yet, look upon the number of leaves, and therefore, the amount of photosynthetic energy required to complete its simple life needs. A plant requires far less energy than a mobile, sentient organism. This is why animals must intake energy in the form of food, instead of converting it from sunlight. Given the approximation of surface ratio to energy in photosynthesis, a high metabolism creature like an Irken, in order to photosynthesize the energy it needed, would have to have something like thirty times the skin surface they have. And since body mass increases with the amount of skin, it would be impossible, even in an environment laden with solar radiation, to achieve this function. So no, Irkens probably are NOT photosynthetic.
The skin color then, is probably more or less a camouflaging feature evolved over time, on the same lines of a grasshopper or praying mantis, both of which share a very similar color to Irkens. To combat the fact that blood (yes, Irkens have to have blood) has color, which would show through in skin (as it does in ours), and once again, if Irk IS receiving more radiation than Earth, Irkens would have undoubtedly long ago evolved thicker skin than we have. If Irk is farther from its sun than Earth, or has a smaller star than the sun, skin would need less pigment as a protective measure against UV light, in which case, the green skin may actually be due to the fact that Irkens may possess green blood. This would coincide with a later theory I state pertaining to Irkens' possible ability to osmose waste from their skin, in which case blood would not only be used to carry oxygen, but waste as well, explaining the green color.