The gill of the horseshoe crab, Limulus polyphemus consists of five paired stacks of thin chitinous lamellae, called book gills, each book containing about 150 lamellae. An oval-shaped region in the center of each lamella is visibly distinct from the outer, or peripheral region and covers about one-third of a lamella. The ultrastructure of these two regions was examined and found to contain morphologically distinct areas. Both sides of the blood space in the peripheral region and the dorsal

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... in the central region are lined with a single layer of thin, undistinguished cells. Cells in the central patch on the ventral, or recovery-stroke side of the blood space, on the other hand, exhibit elaborate infoldings of the basal membrane and interdigitations of lateral membranes with closely-associated and densely-packed mitochondria.