If the bones were solid they would be much heavier and the animals would not be able to move as quickly.
Depends a bit on what you mean. Birds have truly hollow bones to be light enough to fly. Most other creatures have some bones that are kinda-sorta hollow, but filled with bone marrow.
yes EDIT: They are not hollow while they are alive, they are filled with 'living bone' which means that the bone will dry up and decay (like tissue or cartilage) leaving a hollow horn after the animals death.
A full bone is definitely heavier than a hollow bone, hope this helps.
bone marrow..
If you mean "hollow" of a bone... it would be marrow.
No, it is found in the hollow cavity inside the bone.
compact bone
No. Yellow bone marrow is found in the hollow cavity inside the bone.
Fossa
marrow
epiphysis
Coral, jellyfish, and aneomes are hollow bodied animals
hollow bones are stronger than compact bones