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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