The atrial endocardium and myocardium in three teleost species Gadiculus thori J. Smith, Melanogrammus aeglefinus (L.) and Onos cimbrius L. of the family Gadidae are described. The myocardial cells contain few myofibrils and a moderately developed sarcoplasmic reticulum and are connected by short nexuses. G. thori has many atrial specific granules in some of the myocardial cells facing the endocardium. Such cells contain little contractile material. The endocardium is composed of tall cells (10 micrometer) with basal extensions, protrusions of the surface and sometimes cytoplasmic bridges between adjacent cells. The endocardial cells contain bristle-coated vesicles, a well developed agranular endoplasmic reticulum, and many moderately dense bodies (MDB). MDB are membrane-bounded, contain tubules, granules and often fibres, and have different amounts of amorphous material and variable electron density. These bodies appear to fuse in a characteristic manner. Ultrahistochemical tests show that the granules in MDB probably contain catecholamines.

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The ultrastructure of endothelial cells and rodlet cells in the bulbus arteriosus of specimens representing six genera of Cichlidae is described. The former are very closely packed by membrane-bound and mainly electron-dense inclusion bodies (0.34.7 pm).In Apistogramma ramirezi I observed numerous subendothelial rodlet cells throughout the entire length of the bulbus arteriosus. These cells penetrate the endothelium and connect to the latter by desmosomes and tight junctions. The luminal part of the cell contains numerous vesicles and tubules (width SO-IOOnm), whereas the basal part is occupied by a number of membrane-bound, club-like inclusions (length 5 5 pm). Between these two layers there occurs a layer of small, elongated mitochondria. Peripherally, these cells consist of a filamentous wall, except in the apical area.The endothelial and rodlet cell inclusion bodies do not react with phosphotungstic acid (pH I) or Sudan black B stain. The endothelial cells react strongly with periodic acid-Schiff (PAS) stain, whereas the rodlet cells are only moderately coloured by this stain.The present results are discussed and compared with those reported previously for endothelial/ endocardial cells and rodlet cells in bony fish.