Heart Ultrastructure in Daphnia Pulex De Geer (Crustacea, Branchiopoda, Cladocera)

ABSTRACT The heart of Daphnia pulex lacks epi- and endocardium. The myocardium, covered by a thin basal lamina, is not polarized. In relaxed sarcomeres, Z-, A-, I- and H-bands are present. Invaginations of sarcolemma, forming Tz tubules and clefts, are limited to Z-band levels at both sides of the myocardium. Longitudinal tubules branch off from the Tz tubules. The sarcoplasmic reticulum is regular, with transverse tubules at Z- and A-I-band levels which are interconnected with longitudinally directed tubules. Interior couplings of "tubule-totubule" type are located at Z- and A-band levels and peripheral couplings mostly at A-band level. Intercalated discs are present.