The fine structure of the leucocytes of the holothurian, Cucumaria miniata

The fine structure of amoebocytes, lymphocytes, and morula cells is described and related to their functions. Three morphological phases (bladder, transitional, and filiform) of the amoebocyte are distinguished. Their surface protrusions and activities are based on microtubule and microfilament systems and the transitional-filiform phases are functionally involved in coelomocyte aggregation. The bladder phase is phagocytic; bladder formation and activities are also microfilament based. Morula cells contain spherules composed of acid mucopolysaccharide and protein. Dilated rough endoplasmic reticulum (ER) cisternae apparently synthesize spherule material which is added by accretion. Lymphocytes have little cytoplasm and relatively few organelles, except for abundant rough ER and free ribosomes. Lymphocytes are probably stem cells for amoebocytes and morulas. These cells are compared with the leucocytes of other echinoderms.