Natural history and propagation of fresh-water mussels . ulas (PI. XIII, fig. i). 2. Marsupium occupying the entire outer gills, as in tlie heel-splitter, Symphynota complanala (, fig. i). 3. Marsupium occupying the entire outer gills, but differing from the second in that the egg masseslie transversely in the gills, as in the squaw-foot, Sirophitus edentulus. 4. Marsupium occupying only the posterior end of the outer gills, as in the black sand-shell, Lamp-silis recta, etc. (PI. XIII, fig. 2). 5. Marsupium occupying a specialized portion in the middle region of the outer gills, as in th
Natural history and propagation of fresh-water mussels . ulas (PI. XIII, fig. i). 2. Marsupium occupying the entire outer gills, as in tlie heel-splitter, Symphynota complanala (, fig. i). 3. Marsupium occupying the entire outer gills, but differing from the second in that the egg masseslie transversely in the gills, as in the squaw-foot, Sirophitus edentulus. 4. Marsupium occupying only the posterior end of the outer gills, as in the black sand-shell, Lamp-silis recta, etc. (PI. XIII, fig. 2). 5. Marsupium occupying a specialized portion in the middle region of the outer gills, as in thethree-homed warty-back, Obliquaria reflexa (PI. XIII, fig. 3). 6. Marsupium occupying the entire lower border of the outer gills in the form of peculiar folds, asin the kidney-shell, Ptychohranchus phaseolus (PI. XIII, fig. 5). 7. Marsupium occupying the lower border only of the outer gills, Ijut not folded, as in the drome-dary mussel. Dromus dramas (PI. XIII, fig. 4). Most of the commercial species belong to the first and fotirth °—21 5. Fig Horizontal section of a water tube of a gravid marsupium,showing respiratory canals(a. I.), and marsupial space (m. s.), containingglochidia. (After I.,efevre and Curtis.) 140 BULLETIN OF THE BUREAU OF FISHERIES. With such species as have all four gills, or the entire outer gills serving as marsupia,the sexes are scarcely, if at all, distinguishable from an examination of the shell; butwhen a distinct portion of the outer gill is used as a brood pouch there is usually a pro-nounced inflation of the shell over the region of the marsupia, so that the female musselis clearly marked on the exterior. (See also Grier, 1920.) It is to be remarked that the eggs packed into the water tubes or marsupial cham-bers do not usually remain free of each other, but become either attached together bytheir adhesive membranes or else embedded in a common mucilaginous substance. Whenthe eggs or glochidia are removed from the gills they do n
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