The free circulating coelomocytes in the coelomic cavity of echinoderms are considered to be immune effectors by phagocytosis, encapsulation, cytotoxicity, and by the production of antimicrobial agents. Although echinoderms (especially sea urchin embryo) have been used as a model organisms in biology, no uniform criteria exist for classification of coelomocytes in echinoderms, and few studies have reported about the biological functions of their coelomocytes. Hence, we study the coelomocytes in the echinoid sea urchin, Paracentrotus lividus, and describe their morphological and ultrastructural features using light and transmission electron microscopes. We classify the coelomocytes of P. lividus into red spherule and colorless spherule cells, small cells, vibratile cells, and phagocytic cells; petaloid and filopodial cells. To our knowledge, this is the first report describing ultrastructural details of the coelomocytes of P. lividus. J. Morphol. 276:583-588, 2015. (c) 2015 Wiley Periodicals, Inc.