This paper provides the first description of the Haswell cells at the ultrastructural level, as well as giving an insight into their function. Two species of Temnocephalidae were studied, Temnocephala iheringi and T. haswelli. Haswell cells are identical in both species, and their structure indicates that they have a secretory function. They are highly interdigitated with parenchymal cells and are usually joined to them by cup-like desmosomes. Nuclei are irregular, with a honeycomb structure and perichromatin granules. The most prominent organelle is granular endoplasmic reticulum, which is typically arranged in concentric rings that usually encircle a conspicuous Golgi complex. Secretion bodies are secreted via projections of the Haswell cells that reach the surface in the anterior portion of the body and in the tentacles. Distinct pores with a size and distribution consistent with the TEM observations were seen under SEM in these regions.