Fish have no neocortex* and few c-fibres so they cannot have the capacity to feel pain.
Fish have no legs so they cannot have the capacity for locomotor activity
Neither do/can snakes.
Fish have primitive wing like structures called fins, so they can fly a bit.
Ostriches have wings so they can fly.
Ostriches have much bigger wings that swallows so they must fly much better than swallows.
Men pee through their penises (just one each I believe, thank god) ... women have no penis so presumably cannot pee.
So perhaps actual experiment and observation is a better means of determining whether or not a particular animal feels pain, rather than a comparative anatomical approach?
*outer most area of the brain where some people believe humans feel pain.