Hemocytes play vital roles in the immune response. Despite progress in the characterization of
molluscan hemocytes and immune cells, including their cellular receptors and signal transduction
pathways, the processes that lead to their differentiation in bivalve larvae remain unknown. Further-
more, the molecular mechanisms of that decide hemocyte stem cell fate and self-renewal during
development remain poorly characterized. Similar to adult mollusks, the larvae are
fi
lter feeders and are
highly susceptible to pathogens and biotoxins; therefore, it is important to understand the development
and function of their immune system. This review summarizes the current data on the appearance of
elements of the immune system in bivalve larvae. I have discussed why the immune cells emerge before
the circular blood vessel system, which differentiates at the late stages of development. I also discuss
how molluscan hemocytes are involved in the development of disseminated neoplasia.