Quick Question about Nuclear Shell Model

Hi,

Today in my nuclear physics class we started learning about quantum field theory. We learned that pi mesons are the carriers of the nuclear force. We also talked about Shell theory and how its simplified model of potential energy and the Schrödinger Equation give rise to energy levels for nucleons.

From what I understand, individual nucleons can be spinning parallel or antiparallel to the total spin of all the other nucleons. This gives rise to a splitting in energy levels.

This is the part that confuses me: how can mesons carry the force in this interaction of spins when mesons themselves do not have spin?