what is the ball on the bottom of it

Question: what is the ball on the bottom of it
- Keywords:
  ball,
  bottom,
  fen raft spider
Asked by mikeyc to Fen Raft Spider on 23 Nov 2017.
- Fen Raft Spider answered on 23 Nov 2017:
  
  In the picture I’m carrying my egg sac. Lots of spiders, including me, take really good care of our eggs so that they don’t get damaged or dehydrated. I need to carry my egg sac around for weeks in my mouth, not eating, to make sure my babies survive!

Question: what is the ball on the bottom of it