The lymphomyeloid system of nearly adult nurse sharks, Ginglymostoma cirratum, was investigated. Lymphomyeloid structures detectable by the naked eye at dissection are the epigonal organ and the spleen. Microscopic examination shows that the epigonal organ produces granulocytes and lymphocytes. The white pulp of the spleen is lymphoid, whereas the red pulp is mainly erythropoietic. Cells with the morphological characteristics of plasma cells occur in the epigonal organ and the spleen. Peroxidase-positive granulated cells are found in the epigonal organ. In contrast to many other elasmobranchs, the nurse shark lacks the Leydig organ, i.e. the lymphomyeloid structure of the esophagus; but the epigonal organ is well developed and averages 0.60% of the body weight. The spleen weighs about 0.26% of the body weight. The mode of life of the nurse shark in shallow tropical waters probably puts a considerable demand on its immune system. Cells produced by the epigonal organ may be important in immune responses and in inflammatory processes.

In Etmopterus spinax, a small deep water shark, the predominating lymphomyeloid tissue is the so called Leydig organ. This consists of bone marrow-like tissue situated between the muscularis and the mucosa of the esophagus. Examination by light and electron microscopy shows that the Leydig organ produces large numbers of granulocytes and lymphocytes. Two main types of granulocytes occur, tentatively called heterophilic and eosinophilic. The heterophilic cells may be subdivided into three types which differ in the ultrastructure of the granules. Cells structurally resembling mammalian plasma cells are common. The presence of these cells indicates that the tissue is part of the shark immune system.

The ultrastructure of the main lymphomyeloid organs of the holocephalan fish, Chimaera monstrosa, is presented.The thymus is well developed even in adults. It is densely packed with small and large lymphocytes, the former predominating. The ultrastructure of C. monstrosa's lymphocytes is similar to that of mammalian ones. The lymphocytes show intimate connections with large epithelial reticulocytes, which infiltrate the cytoplasm of the lymphocytes with long processes.The lymphomyeloid tissues around the orbit are mainly granulocytopoietic. Two types of granulocytes occur: eosinophilic and heterophilic. The latter is found in excess. The granulocytes are ultrastructurally similar to those of elasmobranchs such as Etmopterus spinax and have similar characteristic granules. In addition to granulocytes in various stages, lymphocytes, plasma cells, blast cells, and solitary macrophages are scattered throughout the tissue.The suprapalatal region also has a similar supply and variety of cells, but most heterophilic granulocytes have modified granules.In the spleen, erythrocytes in different developmental stages are common. This confirms earlier studies which suggested that the spleen functions primarily as an erythrocytopoietic organ.

In the blood of Myxine glutinosa three cell lines may be distinguished: erythro-

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