English: The upper epithelium (region highlighted with a blue bar between panels C and D) faces the water with monociliated cells (example cell bodies highlighted in light blue). Besides their flat appearance, another characteristic of these cells is the presence of dense granules (arrows) that are typically found toward the upper membrane. The intermediate layer (green bar) consists of a mesh of interconnected nonciliated fiber cells (labeled “fc”; one fiber cell and selected extensions highlighted in light green). Arrowheads mark the mitochondrial complex, one of the defining characters of a fiber cell [9]. Fiber cells are contractile and responsible for the relatively fast shape changes of the animal [7,9]. The lower epithelium (orange bar) is mostly made up of monociliated cylinder cells (two examples marked in light red), whose nucleus (labeled “n”) lies characteristically in the proximal half of the cell body, and lipophil cells (highlighted in yellow) that are rich in large vesicles. The lower epithelium layer is responsible for the ciliated movement of the animal, in addition to feeding [9]. Note the consistent thickness of approximately 20 μm for both individuals. Identified cell types of H. hongkongensis are further documented in S1 Fig. Scale bar is 100 μm for (A, B) and 2 μm for (C, D).