REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 129 



ining the marsupium of Margaritana, we have nothing to add to Ortmann's description 

 of this genus, and shall confine ourselves to the Unionidae as restricted by him. 



UnionincB. — In this group there is, as Ortmann has shown, the least amount of 

 differentiation and the structure of the marsupium most closely approaches that of the 

 respiratory gill. Aside from the usual permanent differences, namely, the greater 

 frequency of the interlamellar junctions, their increased thickness and width, and the 

 folding of the glandular epithelium, there is little else to distinguish the marsupial from 

 the respiratory gills in this subfamily. Figure 50, plate xiii, which shows a cross section 

 of two water tubes (w. t.) from the gravid outer gill of QuadriUa ebena, represents the 

 typical appearance in the genera embraced in this subfamily. Only two embryos are 

 drawn in the figure, although actually the water tubes are filled with them. The 

 interlamellar junctions (i. j.) are set very close together, at intervals of about five fila- 

 ments, and the marsupium is capable of only moderate distention. The epithelium 

 covering the inner surface of the lamellae is low and ciliated, while that of the inter- 

 lamellar junctions is high and glandular and exhibits irregular ridges and furrows. 

 The folds of the epithelium are always of course far more pronounced in the non-gravid 

 gill, as in this condition the interlamellar junctions are not stretched as they are when 

 the gill is charged with embryos. The throwing of the epithelium into folds and the 

 bending and crumpling of the septa themselves, when not under tension, is undoubtedly 

 due to the elastic fibers which are wavj- and w-rinkled in the non-gravid gill, while they 

 are drawn out nearly straight when the marsupium is full. 



When highly magnified, as in figure 64, plate xv, the epithelium, resting upon a 

 base of connective tissue and smooth muscle fibers and elastic fibers, is seen to be com- 

 posed chiefly of greatly swollen cells, whose vacuoles are filled with a clear mucus-like 

 colorless fluid. Scattered among these gland cells and seemingly often lying within 

 the vacuoles are seen several smaller and darker nuclei which are the nuclei of leuco- 

 cytes (1). In fact, there can be no doubt that the epithelium becomes infiltrated with 

 wandering blood cells from the underlying blood sinuses in the interlamellar junctions, 

 and many indications are present that seem to show that these cells actually wander 

 through the epithelium into the cavities of the water tubes, but what their ultimate 

 fate is, if this be the case, we are as yet unable to say. There is some evidence that 

 they are ingested by the mantle cells of the glochidia in species that carry the lan'ce 

 over the winter, like Lampsilis, but of this we can not be certain. 



The above description of the epithelium of the interlamellar junctions will apply 

 in essential respects to the marsupium of all of the Unionidae that we have examined, 

 for the same characteristic histological structure is present everywhere. 



AnodoniiiuE. — Ortmann has discovered in the genera which he places in this sub- 

 family a most remarkable differentiation which is evidently an adaptation for increased 

 aeration during the period of gravidity, as it totally disappears after the glochidia are 

 discharged and does not reappear until the onset of the next period. He describes the 

 condition as follows (191 1, p. 324, 325): 



Here each ovisac of tlie gravid female is not formed by a whole water tiibe, but only by a part of 

 it, the middle one, which is separated from two lateral canals by a folding up of the epithelium of the. 



