A field guide to the 17 body plans

Why body plan matters

Before a creature has a single trait, it has a body plan: the basic architecture it is built on. A fish and a worm can both end up small, fast, and silver, but they get there from completely different starting blueprints, and that blueprint constrains everything that follows. Evolution remodels a body plan, it rarely replaces it overnight. A descendant should look like a descendant, so on SpecEvo a lineage usually keeps its body plan and reshapes it under pressure. Choosing the right one up front matters.

SpecEvo offers 17, grouped into three families. You can see the current standout of each on the best of each body plan page.

Vertebrate-style forms

Familiar backboned-creature architectures, and the broad roles they fall into:

• Fish and Swimmer are streamlined bodies built for moving through water, from a classic finned fish to a more generalised aquatic swimmer.
• Tetrapod is the four-limbed land (or land-and-water) body plan, the template behind amphibians, reptiles, and mammals.
• Flyer is anything built around powered flight or gliding, paying the heavy price of getting airborne.
• Grazer, Predator, and Burrower are generalist forms defined by how they make a living: cropping plants, hunting, or living in the ground.

Invertebrates

No backbone, and an enormous range of successful designs, which is the bulk of real animal diversity:

• Arthropod: segmented body, external skeleton, jointed limbs. The blueprint of insects and their kin, the most successful animal design on Earth.
• Crustacean: the armoured, often aquatic arthropods, like crabs and shrimp and their analogues.
• Arachnid: eight-legged hunters and weavers, the spiders and their relatives.
• Cephalopod: soft-bodied, tentacled, and intelligent, the octopus and squid line.
• Gastropod: the shelled or sluglike crawlers that rasp their food, the snails and slugs.
• Worm: long, soft, segmented or simple bodies built for burrowing and sifting.
• Cnidarian: radial, often drifting bodies with stinging tentacles, the jellyfish, anemones, and corals.

Other kingdoms

Life is not only animals. Three non-animal architectures cover the rest of an ecosystem:

• Plant: rooted or anchored producers that make their own food, usually from light.
• Fungus: the decomposers that break the dead back down and recycle nutrients.
• Microbe: the simplest, hardiest life, the microbial mats and films that often form the base of everything else.

Picking one

There is no best body plan, only a best fit. A high-gravity world rewards low, sturdy forms. A sunless ocean rewards chemosynthetic microbes and soft drifters over fast swimmers. A world with no solid ground has no use for a burrower at all. The body plan you choose is the first constraint you hand the engine, and the whole lineage grows from it. If the genre is new to you, what is speculative evolution? is a good primer.