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

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