Answer:
These are the bumpy parts of your digits
What ship has no captain but two mates?
Something you carry while singing.
Blind but cruel.
You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.