Complex sentence: There have been eight ships of the name Hydra in the Royal Navy, named for the Hydra of Greek Mythology, a serpent with many heads (though nine is generally accepted as standard), the centre one of which was immortal.
Split into simpler sentences: There have been eight ships of the name Hydra in the Royal Navy. Each was named after the creature in Greek Mythology, a serpent with many heads (though nine is generally accepted as standard), the centre one of which was immortal.