EEPRODUCTIOX IN THE UXIOXID^ 
81 
they remain without further change in the brood-pouches for 
several months before being set free in the water. 
The glochidium, long thought to be a parasite infesting the gills 
and kno^Ti as Glochidium parasiticum, was proved by Carus in 
1832 to be the larva of the mussel itself, although Leeuwenhoek 
many years before had correctly interpreted its relation. In 1866 
Leydig m.ade the important discovery that the glochidium., after 
leaving the parent, completes its development as a parasite on 
fishes. 
Since an adequate review of the literature relating to the embry- 
onic development and the parasitism of the larva has quite recently 
been published by Harms ('09), an historical account of the sub- 
ject may be omitted here and reference had to his interesting and 
valuable paper. 
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 glo- 
chidia until the discharge of the latter. As no better name seems to 
be available, we shall employ it in this paper. Since the extent to 
which the gills are specialized for this purpose varies in different 
groups of the Unionidae, Simpson ('00), 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 exobranchiae, while those in which the inner 
gills alone receive the eggs are distinguished as endobranchise. All 
of the European and North American species belong to the former 
group, while the latter contains forms that are found chiefly in 
Asia, Australia, Africa, Central Am.erica, and South America. 
As our observations have been confined to the exobranchise, 
reference will be made only to this group, the following subdivi- 
sions of which are recognized by Simpson, each distinguished by 
special marsupial characters. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL.9, XO. 1. 
