instruction:
Given a scientific question, generate a correct answer to it.
question:
Inside each lymph node is a honeycomb of connective tissue with spaces filled by what?
answer:
white blood cells

question:
When pumping blood, the hearts of animals with very long necks must overcome what force?
answer:
gravity

question:
In many polychaetes, the parapodia are richly supplied with blood vessels and also function as what?
answer:
gills