The spleen is a lymphatic organ found in all vertebrates. It is similar in structure to a large lymph node. The spleen serves as a blood storage organ, storing blood during digestion and releasing it when it is finished.
The spleen also makes and destroys erythrocytes (red blood cells). Aging red blood cells are filtered out and broken down, just as in the liver; new red blood cells are generated just as in the bone marrow.