Coelomocytes of the earthworm,Lumbricus terrestris, were studied by transmission electron microscopy. Four morphological cell types are distinguishable: lymphocytic coelomocytes, granulocytic coelomocytes, eleocytes (chloragogen cells), and inclusion-containing coelomocytes. Within these major categories, several distinct cell types differ and may represent developmental stages. The two types of lymphocytic coelomocytes are small with central nuclei and scanty cytoplasms. Two types of granulocytic coelomocytes differ greatly in shape and content; both have small dark-staining granules that resemble lysosomes. Electrocytes, derived from chloragogen tissue, contain a variety of granules, inclusions and vacuoles. Inclusion-containing coelomocytes appear as two types which may be immature and mature forms. Although these cells resemble those that have been referred to as erythroid cells in other invertebrates, the large inclusion bodies are apparently unrelated to hemoglobin; they can undergo morphologic transformation and be extruded by exocytosis. This information on lymphocytic, granulocytic and inclusion-containing coelomocytes is crucial to understanding more about cellular immunity in the earthworm.
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