Evolution of blood cells.
The most primitive blood cell may have been a protohemocyte which was first involved in phagocytosis and nutrition. When metazoans (sponges) appeared, their "blood" cells, the archeocytes, were phagocytic. In diploblastic cnidarians (e.g. Hydra) blood cells occur in the primitive mesenchyme or mesoglea and are still phagocytic. Although the flatworms and nemertean worms possess a pseudocoelom, a progressive differentiation of several leukocytic types occurred. In coelomate annelids or segmented worms, differentiated cells appeared that distributed food and oxygen, thus erythrocytes evolved in certain marine or polychaete annelids. The Annelida also show differentiation of leukocytes that resemble granulocytes, lymphocytes and monocytes, and cell-mediated immunity that persists throughout later evolution. The Mollusca, Arthropoda, Echinodermata and Tunicata possess essentially the same features as the annelids. The vertebrate blood cells may have evolved from the tunicates via the echinoderm line, the deuterostome invertebrates, although analogous blood cells are present in the annelids, molluscs, and arthropods, the protostomes. The vertebrate plan of blood cell differentiation occurs in two compartments: bone marrow (for both lymphoid and myeloid cells) and lymphoid organs (e.g. thymus, lymph nodes) for lymphocytes.
D000818:Animals; D005075:Biological Evolution; D001773:Blood Cells; D004447:Echinodermata; D007448:Invertebrates; D010587:Phagocytosis; D013045:Species Specificity