Answer:
Only two backbones and thousands of ribs.
Long and slinky like a trout, never sings till it's guts come out.
In marble halls as white as milk, lined with a skin as soft as silk. Within a fountain crystal-clear. A golden apple doth appear. No doors there are to this stronghold, yet thieves break in and steal the gold.
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
When the creeper passes, all the grass kneels
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?