REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 13I 



At the outset of gravidity, vertical septa begin to grow out in all of the water 

 tubes of the marsupium from the surfaces of the interlamellar junctions close to the 

 inner and the outer lamellae of the gill. On each side of the gill one septum projects 

 posteriorly, while the other extends anteriorly, and the two meet halfway across the 

 cavity of the water tube. The free edges of each pair of opposed septa then fuse along 

 their entire extent from the ventral border of the gill to the supra-branchial chamber. 

 Specialized elongated epithelial cells forming a serrated border cover the free edge of 

 each septum, and, when the two edges meet, these cells interlock and fuse (fig. 56, pi. 

 xiv). In this way a space, quite narrow and slit-like below, but expanding gradually 

 toward the supra- branchial chamber, is cut off from the water tube on either side, lying 

 between the lamella and the large median division of the tube. As the septa unite, 

 the eggs become confined entirely within the large central space of the original water 

 tube, as Ortmann has stated, and it is this median division alone that functions as the 

 marsupial cavity. We shall speak of the lateral spaces as the respiratory canals, as 

 their function is undoubtedly to conduct a respiratory current of water to the supra- 

 branchial chambers. 



In figure 57, plate xiv, one side of a water tube, with the adjacent portion of the 

 lamella, taken from a gravid marsupium of Anodonta cataracta, is shown in horizontal 

 section. The gill contains eggs in an early cleavage stage, only four of which, however, 

 are represented in the figure. The septa (s) are seen approaching each other, having 

 not yet quite met. In figure 51, plate xiii, taken from the same species but not so 

 highly magnified as the last figure, the septa have fused and the respiratory canals 

 (r. c.) are completely shut off from the marsupial space (m. s.). In both of these figures 

 the sections were taken near the ventral border of the gill; had they been cut at a 

 higher level, the canals would be seen as much larger spaces. As is clearly shown in 

 figures 56 and 57, plate xiv, the ostia open freely into the respiratory canals, and water 

 must therefore enter the latter directly from the mantle chamber. The condition 

 here should be contrasted with that seen in figures 50 and 53, plate xiii, which show 

 water tubes from the marsupia of Quadrula and Lampsilis, representatives of Ortmann's 

 Unioninae and Lampsilinae; here the ostia lead directly into the cavity of the tubes 

 (w. t.) which are not subdivided and the whole of which becomes filled with eggs. 

 Although it is not shown in figures 51, plate xiii, and 57, plate xiv, the epithelium cov- 

 ering the outer wall of the canals, which is of course the lining of the lamellae, bears 

 cilia which probably aid in conducting the current of water toward the suprabranchial 

 chamber. Below, the canals are closed, and, since they are shut off from the marsupial 

 cavity after the fusion of the septa, but open freely above into the suprabranchial 

 chamber, there is but one course for the water to take — it must pass upward and 

 enter the suprabranchial chamber. The transition from the more or less flattened 

 epithelium lining the outer and inner walls of the respiratory canals to the large colum- 

 nar cells on the anterior and posterior surfaces is clearly seen in figure 57. 



The same condition appears in figure 58, plate xiv, which shows one end of a canal 

 (the end marked X in the preceding figure) and the adjacent tissues, but under a higher 

 magnification. Among the columnar cells are seen numerous swollen mucus cells, 



