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The planktonic  Oikopleura dioica  belongs to Tunicata, the probable sister taxon to Craniota, and might show plesiomorphic characters, conserved from the common lineage of Tunicata and Craniota. In  O. dioica  a pericardium in a position similar to other chordates but also to the heart and pericardium of craniates is found. Surprisingly, little is known about the ultrastructure of the pericardium in  O. dioica . Here, we show based on electron microscopy that the pericardium is completely lined by a single layer of 16 epithelial cells: 6 epithelial myocardial cells on the left side of the pericardium and 10 peritoneal cells constituting the right side. One of the peritoneal cells, situated at the ventral border between peritoneal cells and myocardial cells has an extension that anchors the pericardium to the basal lamina beneath the latero-ventral epidermis. The primary body cavity of  O. dioica  appears quite uniformly clear in electron microscopic aspect but several sheets, resembling the basal lamina of the pericardium cross the larger spaces of the body cavity and connect to the pericardial basal lamina. This is the first detailed description of two distinct cell types in the epithelial lining of the pericardium of  O. dioica.  In comparison with other chordates, we conclude that two cell types can be reconstructed for the last common ancestor of Chordata at least. The position of the pericardium at the intersection of trunk and tail in combination with the basal-lamina like sheets spanning the hemocoel is probably of importance for the function of the circulation of the hemocoelic fluid. Similar to the tail, the axis of the pericardium is shifted through 90 degrees to the left as compared to the main body axis of the trunk and we infer that this shift is an apomorphic character of Appendicularia.

The pericardium of Oikopleura dioica (Tunicata, Appendicularia) contains two distinct cell types and is rotated by 90 degrees to the left