Ultrastructural studies of the hearts in Arenicola marina L. (Annelida: Polychaeta)

Summary The two hearts in Arenicola are capable of great dilation and contraction. The heart wall consists of myoepithelial cells resting on a basal lamina. On the luminal side of the basal lamina is a layer of collagen fibrils. No true endothelium was observed but occasional haemocytes were observed, subjacent to the collagenous layer. A few chloragogen cells are also found peripherally.The myofibrils are of a non-striated type consisting of thick and thin filaments and scattered Z-bodies. The sarcoplasmic reticulum forms a three-dimensional network. Only peripheral couplings were observed. The myofibrils are in contact with the sarcolemma on the luminal side of the cells, constituting a kind of hemidesmosome. The myoepithelial heart muscle is compared with other muscle types described in invertebrates. Supercontraction is discussed.