REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 33 



supium of Symphynota complanata at a stage when the glochidia are fully formed. In 

 this species, when the marsupium is fully charged, the interlamellar junctions are so 

 stretched that they become greatly reduced in thickness and appear quite membranous. 

 Figures 49 to 53, plate xiii, showing a gravid water tube in Alasmidonta, Oiiadrula, 

 Anodonta, Symphynota, and Lampsilis, respectively, are all drawn under the same 

 magnification, and should be compared in order to observe the relative sizes of the 

 tubes in section in the several cases, as well as the different inter\'als between the inter- 

 lamellar junctions as shown by the number of intervening filaments in the lamellae. 



Ortmann interprets the respiratory canals of the Anodontinae as an adaptation for 

 the better aeration of the embryos in the marsupium (191 1, p. 325). They are unques- 

 tionably a respiratory device, but for many reasons it would seem clear that they serve 

 primarily for the aeration of the blood of the gravid female and not of the embryos. It 

 is difficult to see how a membrane which shuts the embryos off from the water could 

 increase the facilities for aeration or why such a condition should be an improvement^ 

 as far as the embryos are concerned, over the marsupium in those genera where there are 

 no respiratory canals and the water conies into direct contact with the embryos. In 

 some of the species of Lampsilis {ligamentina, for example) the marsupium is as heavily 

 charged as in many of the Anodontinae, and the glochidia are also carried over the winter, 

 yet the respiratory canals are not present. In either case the embryos probably receive 

 an adequate amount of oxygen. .But, on the other hand, it is not difficult to see that 

 the respiration of the gravid female might be seriously interfered with, when the entire 

 outer gill is gorged and swollen with glochidia and these same glochidia must remain in 

 the marsupium for months. In the Unioninae (Ortmann) the marsupium is gravid for 

 only a few weeks at the longest, and, furthermore, the gills are not so heavily charged, 

 while in the Lampsilinee only a differentiated portion of the outer gill receives the embryos 

 and, although the marsupium may be heavily loaded and remain gravid over the winter, 

 the encroachment of the marsupial upon the respiratory function is not so extensive. 

 In these two subfamilies the need of a special respiratory device is, therefore, not as 

 great as in the Anodontinae. The close association of the maternal blood with the cur- 

 rent of water in the respiratory canals, as shown in figure 58, plate xiv, would add further 

 evidence for the view that the secondary division of the water tubes is an adaptation 

 for the better aeration of the blood of the gravid female, in correlation with the prolonged 

 period of gravidity and the interference with respiration by the excessive crowding of 

 the entire outer gill. 



Reference should be made to the special conditions existing in Sirophitus. Aside 

 from the formation of the respiratory canals in the manner peculiar to the Anodontinae, 

 Ortmann has brieflv described a division of the marsupial ca^-ity of each water tube by 

 the outgrowth of horizontal septa from the interlamellar junctions to form separate 

 closed spaces each one of which incloses a single " placentula " Referring to the pecuHar 

 position of the "placentulae," which lie crosswise in the gill, he says (191 1, p. 294): 



This arrangement is brought about by further outgrowths of the epithelial layers of the septa (inter- 

 lamellar junctions), which fill the spaces between two septa, or rather only the middle part, the ovisac. 



