REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 39 



either all four gills or only the inner or outer ones, or, again, only a portion merely ot 

 one or more gills. In the pre-glochidial stages, when the embryos are conglutinated, 

 the entire masses were discharged, while individuals were frequently seen in the act of 

 aborting their embryos or glochidia which were often expelled with considerable force 

 through the exhalent siphon. 



This behavior was so characteristic of the genus that, in order to make a correct 

 determination of the condition of the marsupium, it was necessary to open quadrulas 

 immediately after taking them from the water. When this was done, all four gills 

 were invariably found to be charged on opening females which contained embryos in 

 pre-glochidial stages — -that is, at any time before normal spawning had occurred. The 

 habit of readily aborting embryos when disturbed has also been observed by us in 

 Unio complanalus , which has been repeatedly seen in the act of discharging the contents 

 of the marsupium shortly after being placed in aquaria. In all likelihood it occurs in 

 other species of Unio, and it may possibly be characteristic of all forms in which there 

 is but Httle structural differentiation of the marsupium. We have, however, also 

 observed the discharge of embryos in Lampsilis ligamentina, but only after the gravid 

 females have been kept in the laboratory for some time. This species is apparently 

 very much less sensitive with respect to abortion than the quadrulas and Unio com- 

 planalus and only frees its gills of the conglutinates after long exposure to artificial 

 conditions. The premature extrusion is probably due to imperfect aeration of the water 

 and results from an effort on the part of the female to secure more oxygen; if this be 

 true, one would not expect to find it occurring so readily in those forms which have a 

 differentiated marsupium, hke the Heterogenae, since here the respiratory and marsupial 

 functions of the gills are not so intimately associated. 



Both Schierholz (1888) and Latter (1891) have referred to the occurrence of 

 abortion in Anodonta, but according to our experience it has never been encountered 

 in a single instance in either Anodonta or Symphynota, although gravid females have 

 been kept in tanks in the laboratory for weeks or even months. The presence of the 

 respiratory canals, which have been described as occurring in these genera during 

 gravidity, as well as the temporary membrane which roofs over the marsupial division 

 of the water tubes, might well account for the absence of abortion, or at least its rare 

 occurrence, in the forms in which these special conditions exist. The respiratory canals 

 doubtless lessen the evil effects of poor aeration, while the roofing membrane of the 

 water tubes would certainly offer some obstruction, as long as it was present, to a 

 Uberation of the embryos. 



BREEDING SEASONS. 



In connection with our study of artificial propagation of fresh-water mussels, we 

 have found it necessary to collect data bearing upon the breeding seasons of a fairly 

 wide range of species, since the records of previous observers, for North American 

 Unionidte at least, have been insufficient to enable us to determine the full extent of 

 the seasons, especially in the case of some of the more important commercial species. 



