47 
common oyster , and the large fresh-water muscle. 
appear when floating in water, when they assume nearly a 
spherical form : at a is one of the ova after having been about 
one hour in water, when they dissolve into a granulated mass ; 
magnified 100 diameters. 
Fig. 4. An equally small portion of an ovarium in a more 
advanced state of pregnancy ; magnified 100 diameters. 
Fig. 5. Some ova extracted from the above portion of 
ovarium, floating in water, magnified 100 diameters: at a is 
one of these ova bursting, and emitting its granulated sub- 
stance ; magnified 100 diameters ; at b is an empty, unim- 
pregnated probably, ovum ; magnified 100 diameters. 
Fig. 6. Several young oysters just emitted from the oviduct, 
and floating in water and their natural mucus ; magnified 100 
diameters. 
Fig. 7. Some of the same young oysters, after having been 
some time in contact with the air, when they become quite 
opaque and assume a glossy appearance ; magnified 1 00 
diameters. 
Fig. 8. A small portion of the cloak with its fringe ; mag- 
nified 25 diameters. 
Fig. 9. A portion of the fringe spread open ; magnified 50 
diameters. 
Fig. 10. The heart, with its auricles in the natural position, 
as it lies in the oyster ; magnified 4 diameters. 
Plate V. Fig. 1. A fresh- water muscle, one shell, and its 
lining or cloak removed ; natural size. 
Fig. 2. The same muscle with the oviduct removed, and a 
perpendicular section of the ovarium ; natural size. 
Plate VI. Fig. 1. A very small portion of the ovarium, with 
