Semi-thin sections of the sac-like structures from Pseudopleuronectes obscurus. a) In the central region of the sac, X-cells (x) are found to be mostly restricted to large masses within the host epidermal tissue contained by a thick basal lamina-like membrane (black arrowheads), under which lies connective tissue. Some X-cells inside the large aggregations appear to be degenerate (black arrows), whilst others can be found amongst the connective tissues (white arrows) outside of the membrane that contains the majority of the X-cells. b) At the border of the sac there is a less organised tissue structure observed. Aggregations of X-cells (x) are no longer contained within membranous structures and numerous X-cells are present at the peripheral margin of the sac (black arrowheads) and some appear to be independent from the sac (white arrows). Scale bars 20 μm.