What is the function of blood glands of an earthworm?

So, blood glands are located in the 4th, 5th and 6th segments. My textbook says that blood glands produce blood cells and haemoglobin( which is dissolved in plasma). It also says that blood cells are phagocytic (like WBCs).
My question is that these blood cells must be nucleated, so why do they need blood glands to produce them, why don't they divide themselves?

Question

What is the function of blood glands of an earthworm? 

So, blood glands are located in the 4th, 5th and 6th segments. My textbook says that blood glands produce blood cells and haemoglobin( which is dissolved in plasma). It also says that blood cells are phagocytic (like WBCs). 
 My question is that these blood cells must be nucleated, so why do they need blood glands to produce them, why don't they divide themselves?