Carcinization

[deckardcanine]

The Internet quips about carcinization,
Where critters evolve to have crab body plans.
"Will we become like that?" "We already have:
We lost tails, can walk sideways, and grip with our hands!"

Biologists' standards, of course, aren't that simple.
A crab needs a carapace flattened and broad,
A flat and bent pleon, and sternites fused into
A wide sternal plastron. And yes, that sounds odd.

The point is, it's happened five times that we know of.
Quite likely a sixth involves cycloids, extinct.
The versatile plan lets crabs go where the prior
Crustaceans could not. Perhaps that's how they're linked.

But lest you believe it's the ultimate body,
Decarcinization occurs even more.
My takeaway: Nature rewards flexibility,
Letting us change for whatever's in store.