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Physical and cytochemical techniques have demonstrated that in Spirographis spallanzanii there is a highly pigmented tissue surrounding the ventral and circular vessels. Melanin and carotenoids are absent in the clubshaped cells, but there is evidence for the presence of lipids, haemopigments and Fe+++, probably in connection with haemopoietic processes. Tests on the ala-dehydrase activity in this tissue confirm this hypothesis. From these results it can be concluded that in Spirographis spallanzanii this tissue is the site of synthesis of respiratory pigment.