Critter Crafting, Part 1 of 3 – The “Universal Core”

Since I’d addressed ecosystems in the previous post, I thought I might as well look at the design for individual creatures. This will be a three-part series, of which this will probably be the longest. They’ll look at the universal core, animal variation, and plant variation.

Important note: I’m not a professional biologist. I’m just a hobbyist who studies the field to make interesting worlds. If you have any reason why anything I write might be inaccurate, please let me know.

To start with, I’m going to introduce a concept that I call the “universal core.” This is a structural layout that most large creatures will share (here “large” roughly means “bigger than a German Shepherd”). Let’s dive into what the universal core looks like.

The Basic Layout

The “core,” or head and torso

The basic structure of the core is pretty simple:

  • Head – An appendage with sensory and nervous centers as well as entrance orifices. Can be attached by a neck.
  • Torso – A central trunk that has the hub organs of all other systems.
    • Ribs – A relatively immobile portion that protects delicate organs.
    • Gut – A relatively flexible portion that contains resilient organs, as well as those that might change significantly in size.
    • Rear – Exit orifices opposite the head.

Those are the absolute basics, and if you want, you can leave it at that. We can get way more specific, though. Let’s start with organ systems.

Systems of the Body

Some of our many organs

In order for most creatures to function, its cells need to specialize to accomplish different functions. These will form hierarchies of organs and other tissues. Here are the systems that are almost universal:

  • Skeletal – There needs to be something to give the creature structure. For animals of any size, this is usually an endoskeleton. Exoskeletons and more exotic variants (like the lattice skeleton of sea stars) don’t work as well scaled up.
  • Muscular – Obviously, the creature needs to move. There aren’t many variations on the traditional muscular system.
  • Circulatory – Resources need to be distributed throughout the body. One or more central pumps (hearts) connected to a series of tubes (blood vessels) is the most efficient.
  • Nervous – The creature needs to be able to sense its environment, make decisions, and transmit orders to the body. If the creature’s of a decent size, delegating these functions through a decentralized nervous system probably won’t work; a central processing organ (brain) will be used.
  • Digestive – There needs to be a way to process resources collected from the environment. This will want to maximize available surface area, so a long series of tubes (intestines) will be used. One or more stomachs will be used to “pre-treat” the ingested food.
  • Respiratory – Assuming that there will be nutrients in the air, the creature will need a respiratory system. The distributed systems that are available for insects and other smaller creatures don’t work well at larger sizes; lungs and mouth(s) are the only sensible option.
  • Reproductive – Sexual reproduction isn’t the only viable method, but it needs to reproduce somehow.

There are supportive systems as well (such as the endocrine), but these are the big ones.

Putting it All Together

Model of a deer’s organs for target practice

Now that we have the layout (the core) and the organs, we can get into specifics regarding what goes where. What does it all look like?

  • Head
    • Nervous – The basic purpose of the head is to house sensory organs and the brain. Complex sensory information (sight and hearing especially) is hard to transport over long distances. Since that’s the case, putting the organs as close to the brain as possible is best.
    • Skeletal – The brain is the most delicate organ of the body, and doesn’t need to change size to operate; putting it in a skull is only natural. There are pros and cons to having a neck: on the plus side, it allows the head to reorient to gain more sensory information; the con is that it’s a major structural weakness.
    • Entrance Orifices – If there’s a hole for something to go into the body (respiration or digestion), it will probably go here. This is for two reasons. The first is that they often process sensory data (smell and taste respectively), so they should be close to the brain. Two, gaining as much information as possible about what the creature is ingesting is valuable, so having the entrances near the other senses is good. The respiratory and digestive tracks can briefly connect to save space, but it’s not always necessary; horses’ mouths don’t connect to their lungs, for example.
  • Torso
    • Skeletal – The torso will probably have a spine with vertebrae, which allow for maximum flexibility.
    • Ribs
      • Skeletal – The defining feature of this area is the ribcage. This protects organs that, if damaged, could very quickly lead to death. In a perfect world, you wouldn’t need a ribcage at all, since it makes that portion of the torso pretty stiff. Making it out of ribs instead of a solid shell helps a bit, but not a lot.
      • Delicate Organs – Circulatory and respiratory hubs fall under the heading of “hurt them and you die fast,” so they go here. Lungs expand and contract, which is why we still need the flexibility that ribs offer.
    • Gut
      • Skeletal – This portion of the torso has a spine, but no ribcage. This is where you put the organs that won’t lead to an immediate death if injured. Conveniently, these also need room to expand if necessary.
      • Hardy Organs – Ruptured intestines clearly aren’t good, but aren’t necessarily fatal. They also need the ability to swell in case of a large meal. If the creature gives live birth, the uterus also needs to go here for obvious reasons.
    • Rear
      • Exit Orifices – Everything that expels waste should be as far from the entry as possible to prevent contamination. Reproductive organs could also go here to take advantage of pre-existing holes.


Representations of the three main exceptions to the universal core

As much as I like to throw around the world “universal,” it’s obvious that there are a lot of animals that don’t have this core. There are three main conditions in which there would be exceptions:

  • Aquatic – Water significantly changes the game, lessening gravity’s effects in exchange for pressure. Extremely bizarre body plans are possible at large sizes; see the giant squid above. A sufficiently dense atmosphere could have the same effect.
  • Small size – Size has a much greater effect on creature design than many people realize. If you get small enough, almost anything is possible. See the exoskeletal crab above; exoskeletons are one thing that aren’t practical at large sizes. The coconut crab is the largest thing with an exoskeleton nowadays.
  • Chemical availability – Sometimes, if different chemicals are available to build the creature from, different body plans become practical. At one point in Earth’s history, the atmosphere had a lot more oxygen. With that, larger exoskeletons became possible. The arthropleura at top right was the largest bug in history at three meters long; it couldn’t survive in today’s environment.

And that’s all for today! I’d love to hear any feedback or suggestions. See you soon with part 2!

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