Platypuses are monotremes and their nearest relatives are Echidnas.
The platypus is not related to birds. They are monotremes. (mammals that lay eggs.)
For a map of where the platypus can be found, see the related link.
Several diagrams of the platypus - both external and internal - can be found at the related link below.
No other animal looks like a platypus. It is completely unique.
A platypus's nostrils are positioned towards the end of its bill, not at the top. See the picture at the related link below.
The photograph at the related link below shows a male platypus's spur on its hind foot.
The spiny anteater or echidna is most closely related to the platypus.
You can see photos of the male platypus's venomous spur at the related link below.
The echidnas are the only living relatives of the platypus.
They have tails.
There is very little available out there that speaks to the anatomy of a platypus. The related link below will show a general picture of the brain, as well as written material about the rest of the internal anatomy.
Neither. A platypus is totally different from either - it lays eggs and has a bill that does not actually even resemble the bill of a duck, as it has completely different functions. The platypus is an egg-laying mammal, or monotreme, in the same family as e echidna. it is not related to the beaver, which is a placental mammal; nor is it related to the duck, which is not a mammal at all, but a bird.