Why do poliwag, poliwhirl, and poliwrath have a swirl on their bellies?

Question

This is your present Red...

Answer 1

This is what the Pokemon Stadium Pokedex entry says about Poliwag

"The spiral on its belly is its internal organs seen through its thin skin. It is better at swimming than walking."

So yeah, the spiral is their crazy amphibian digestive tract. Childhood ruined.

But there is a legitimate reason for this. Many frogs in real life have see-through skin, so this is likely the inspiration.

Example: http://alexpb.com/notes/wp-content/uploads/2009/09/apr08-02-1280.png

Hope this helps

Answer 2

Ahem, Pokedex entries:

>The direction of its belly spiral differs by area. The equator is thought to have an effect on this.
The swirl on its belly is its insides showing through the skin. It looks clearer after it eats.
POLIWAG has a very thin skin. It is possible to see the POKéMON’s spiral innards right through the skin. Despite its thinness, however, the skin is also very flexible. Even sharp fangs bounce right off it.
It is possible to see this POKéMON’s spiral innards right through its thin skin. However, the skin is also very flexible. Even sharp fangs bounce right off it.
Its slick black skin is thin and damp. A part of its internal organs can be seen through the skin as a spiral pattern.
Its newly grown legs prevent it from walking well. It appears to prefer swimming over walking.
Its skin is so thin, its internal organs are visible. It has trouble walking on its newly grown feet.

Yeah, so it's the organs, and the skin is thin.

Hope this helps!

Answer 3

My Somewhat Illogical Theory

Well, yes. The reason it's belly has a swirl on it is because of it's thin skin, and that is it's internal organs. :3

Like Lombro said, this seemed inspired by real-life frogs. However, I think the main reason it's internal organs are a big swirl is to relate to the hypnosis attack it knows.

Lombro's picture shows a frog with it's internal organs showing - and they don't look like a big swirly line. In fact, it looks more like a giant-sized atom. (xD)
I think the reason they decided to make it a swirly line was:
a) Seeing that sort of thing wouldn't appeal to children

b) It was hard to draw with the limited resources to create detailed sprites in Gen I(I)

and/or

c) It would be it's main source for creating Hypnosis.

So, that's what I think.
Hope I helped. :)
Sources: Previous answers