The heart-body in Neoamphitrite figulus (Dalyell) forms a mass of tissue within the supra-oesophageal vessel almost occluding the lumen. The tissue forms a much-infolded cylinder enclosed by a basal lamina consisting of a fibrous reticulum through which the assembled haemoglobin molecules are discharged into the plasma (Dales & Pell, 1970). A function of both the heart-body and the extravasal (‘chloragogen’) tissue in polychaetes without a heart-body was established by biochemical analysis (Kennedy & Dales, 1958; Dales, 1963, 1965) to be the synthesis of plasma haemoglobin (erythrocruorin) or chlorocruorin. This was confirmed by electron microscopy (TEM) by Breton-Gorius (1963) in Arenicola, Potswald (1969) in Spirorbis, Dales & Pell (1970) in Neoamphitrite, Lattice, Myxicola, Megalomma and Sabella, and by Friedmann & Weiss (1980) in Amphitrite. It would appear that the same tissues also sequester phagocytosed materials (Braunbeck & Dales, 1984). Re-examination of this tissue by trans-mission electron microscopy (TEM) has extended our knowledge of the ultra-structure. Here we discuss these results in relation to the function of these tissues in the production of the respiratory pigments found in the plasma. Some heart-bodies of Terebella lapidaria L., Lattice conchilega (Pallas) and Cirriformia tentaculata (Montagu) have also been examined by TEM for comparison with that of Neoamphitrite figulus.