The ultrastructure of book lung development in the bark scorpion Centruroides gracilis (Scorpiones: Buthidae)

In scorpion embryos, there is ingression (inward migration) of atrial hypodermal cells rather than invagination or infolding of the atrial hypodermal layer. The ingressing cells proliferate and align in rows anterior to the atrium. Their apical-basal polarity results in primordial air channels among double rows of cells. The cuticular walls of the air channels are produced by secretion from the apical surfaces of the aligned cells. Since the precursor cells are in rows, their secreted product is also in rows (i.e., primordial air channels, saccules). For each double row of cells, their opposed basal surfaces are gradually separated by a hemolymph channel of increasing width.The results from this and earlier studies show there are differences and similarities in the formation of book lung and book gill lamellae. The homology hypothesis for these respiratory organs is thus supported or not supported depending on which developmental features are emphasized. For both organs, when the epithelial cells are in position, their apical-basal polarity results in alternate page-like channels of hemolymph and air or water with outward directed hemolymph saccules for book gills and inward directed air saccules for book lungs.At the end of the ninteenth century and in the early twentieth century numerous papers were published comparing the development of book gills in the horseshoe crab with the development of book lungs in arachnids, especially spiders. As reviewed by Farley [1], this work was done with the hypothesis that these respiratory structures are homologous, e.g., the internal book lungs in the opisthosoma were derived by insinking of the anlage that had previously resulted in external book gills of an aquatic ancestor. There have been diagrams and much discussion about how an ancient ancestor with lamellate gills like extant horseshoe crabs could have given rise to arachnid book lungs [2-12]. Recent investigations of horseshoe crab, scorpion and spider embryos report si