The present paper considers ascidian cellular defence reactions, giving an account of the role played by each hemocyte type. The blood cells involvement occurring in the processes of wound healing, coagulation, phagocytosis, encapsulation, histoincompatibility, and cytotoxicity is reported and discussed. With the aim of making comparisons and pointing out cell functions, a simple and unitary classification scheme of the hemocytes is proposed. Morphofunctional identikits of the hemocyte categories in ascidians are presented.