The primary function - is to contain the developing embryo and its nutritional yolk-sac until the foetus is ready to emerge. That applies whether it's a hard-shelled egg like a bird, or a soft-shell like a reptile.
The egg of a platypus is leathery. It is not hard-shelled like a bird's egg.
to protect them from pridators
A bluebird.
Yes.Platypuses, along with echidnas, are the only known egg-laying mammals, or monotremes. They lay soft-shelled, leathery eggs.
Reptile eggs are soft-shelled - as opposed to the calcified shell of a birds egg.
The eggs laid by monotremes (egg-laying mammals) are still called eggs. They are soft-shelled and leathery, rather than hard-shelled like birds' eggs.
Reptile eggs are soft-shelled so that the babies can break out of the shells. Bird eggs are hard-shelled, because baby birds use their beaks to crack the shells of their eggs, but since reptiles don't have beaks to use to break their shells, their eggs have to be soft-shelled.
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No. Both species of echidnas, the short-beaked and the long-beaked echidna, lay soft-shelled, leathery eggs. This is the same as the platypus, the other monotreme, or egg-laying mammal.
Yes. Most farms will use the soft shelled eggs for themselves as they do not keep well and the lack of calcium in the shell makes them hard to store. There is nothing wrong with the "rubber" egg except the hen can no longer process the calcium she ate for use in the making of an egg, or is not receiving enough calcium in her diet. It's a good idea to feed hens the shells from their eggs, dry and crushed, as a calcium supplement.
They are naturaly shelled when they are grown
"Shelled" is not a verb on its own. It is typically used as an adjective to describe something that has had its shell removed, like "shelled nuts" or "shelled peas."