The fine structure of blood follicles in the earthworm genera Amynthas and Lumbricus (Annelida: Oligochaeta)

Blood follicles of the earthworm Amynthas are hemoglobin‐containing, sac‐like dilatations of blood vessels which connect to the general circulation. Grape‐like clusters of follicles are found posterior to the pharynx, among tufts of micronephridia, and single follicles are located among cells of the pharyngeal gland. In Lumbricus , follicles take the form of simple swellings and irregular‐shaped diverticula of nephridial capillaries. The fundamental structure of the wall of follicles and of vessels in both genera is the same and consists of two layers: an extracellular vascular lamina and an outer (coelomic) covering of smooth muscle‐like myoperithelial cells. Hemocytes may be free and circulating or they may facultatively attach to the vascular lamina as littoral cells, constituting an incomplete endothelium‐like surface. Hemocytes that appear to be in the process of attaching or detaching are rounded, while adherent cells are flattened and elongate. Free and littoral hemocytes actively endocytose packets of circulating extracellular hemoglobin. Hemocytes within follicles possess radiating cell processes which also endocytose hemoglobin. Although these cells were presumed to secrete hemoglobin, staining with 3,3′‐diaminobenzidine confirms the presence of hemoglobin only within pinosomes and not within protein‐synthesizing or packaging organelles. The presence of hemosiderin‐like bodies suggests that follicular hemocytes catabolize hemoglobin. Blood follicles apparently provide a means of significantly increasing cell‐surface area for hemoglobin processing, without substantially increasing the volume and pumping load of the circulatory system.