116 BULLETIN OP THE BUREAU OF FISHERIES. 



become filled with eggs, a directive mechanism is probably absent in this genus, and a 

 careful comparison of the conditions in Ouadrula with the structure of the g^lls in 

 those genera in which only a portion of the gills is utilized as a brood chamber might 

 well furnish the clue to the discovery of a special mechanism in the latter. 



While as a rule the great majority of the eggs, when a gravid gill is examined, are 

 found to be fertilized, different species differ markedly in the percentage of unfertilized 

 eggs present, and, in fact, a large proportion of the latter seems to be characteristic of 

 certain genera. In Lampsilis , Symphynota, Anodonta, and a number of other genera it 

 has been very imusual in our experience to encounter any considerable number of unfer- 

 tilized eggs, while, on the contrary, in Quadrula, Plcurohcma, and in some species of 

 Unio it is often true that even a majority of the eggs in a gravid female have failed of 

 fertihzation ; in fact, in these genera one expects to find a large percentage of such 

 eggs as the usual thing. 



The entire embryonic development takes place in the gills of the female, and at 

 the close of this period the larva or glochidium is fully formed. The differences in the 

 length of time the glochidia are retained in the gills will be discussed later, but after 

 their liberation the completion of their development occurs while they are living as 

 parasites on the fish in all of the Unionidae, so far as known, except in the genus Stro- 

 phiius, whose glochidium, we have recently discovered, undergoes the metamorphosis 

 in the entire absence of a parasitic stage. This extraordinary case will be referred to later. 



As the embryology of the Unionidae has been described by Lillie (1895) in great 

 detail, and as Harms (1909) still more recently has published an excellent account of 

 the post-embryonic development, we shall omit all reference to the actual develop- 

 mental events, and confine ourselves to a discussion of those phases of the reproduction 

 and parasitism of the Unionidae in which we have been especialliy interested in connec- 

 tion with the problem of artificial propagation. 



THE MARSUPIUM. 



The term marsupium has been generally used to indicate those portions of the 

 mussel's gills into which the eggs are received from the suprabranchial chambers after 

 ovulation and which serve as brood pouches for the retention and nurture of embryos 

 and glochidia until the discharge of the latter. As no better name seems to be availa- 

 ble, we shall employ it in this paper. 



USE OF THE MARSUPIUM IN CLASSIFICATION. 



Since the extent to which the gills are specialized for this purpose varies in dif- 

 ferent groups of the Unionidae, Simpson (1900), in his "Synopsis of the Naiades," has 

 made use of the marsupium as the chief diagnostic character on which his classification 

 is based. Those groups in which the marsupium comprises the outer or all four gills 

 he designates as the Exorbranchiae, while those in which the inner gills alone receive 

 the eggs are distinguished as the Endorbranchise. All of the European and North Ameri- 



