Echinoderms such as urchins are important in marine ecosystems, particularly as grazers, and unhealthy urchins can have important ecological implications. For instance, unexplained mortalities of Diadema sp. in the Caribbean were followed by algal overgrowth and subsequent collapse of coral reef ecosystems. Unfortunately few tools exist to evaluate echinoderm health making management of mortalities or other health issues problematic. Hematology is often used to assess animal health in many animal groups including invertebrates but is seldom applied to echninoderms. We used a standard gravitometric technique to concentrate fixed ceolomocytes from the collector urchin Tripneustes gratilla onto microscope slides permitting staining and enumeration. Using Romanowsky stain and electron microscopy to visualize cell details, we found that in addition to amoebocytes, vibratile, clear and red spherule cells, Tripneustes has at least three other types of coelomocytes. Moreover, cytophagia of host cells by less than 1% of circulating amoebocytes is common (seen in 71% of sea urchins sampled). Cytophagocytic amoebocytes seems to target mainly the motile cells including red spherules, clear spherules, and vibratile cells disproportionate to underlying populations of these cell types. Lectins appear to bind to coelomocytes selectively and could be a useful biomarker for identifying or purifying echinoderm coelomocytes. The blood collection and smear preparation methods described here are simple, field portable, and might be a useful complementary tool for assessing health of other marine invertebrates and revealing heretofore unknown physiologic phenomena in this animal group.