Complex sentence: The most well-known "contrade" are probably the 17 that race in the Palio di Siena-each named after an animal or symbol and each with its own long history and complicated set of heraldic and semi-mythological associations.
Split into simpler sentences: The most well-known "contrade" are probably the 17 contrade of Siena that race in the Palio di Siena. Each is named after an animal or symbol and each with its own long history and complicated set of heraldic and semi-mythological associations.