The response of the various types of defense mechanisms of marine invertebrates after exposure to environmental pollutants have been observed histopathologically. Two of the most important types are the mucous defense system and the cellular defense mechanism. The comparison of types of amebocytes that are part of the cellular defense mechanism, their origin and multi-potentiality were studied. These studies indicate that the kind of amebocytic response may depend on the type and degree of stress present and on the species being studied.