94 GEORGE LEFEVRE AND WINTERTON C. CURTIS 
a time, such eggs generally become swollen and stratified into 
three distinct layers, a heavier, often pigmented, mass at one pole, 
a clear or hyaline intermediate zone, and a small granular cap at 
the lighter pole. As the eggs lie in a constant position in the gills, 
which are placed vertically in the normal position of the animal 
it cannot be doubted that the stratification is produced by 
gravity. It has not yet been determined whether the substances 
which occur in these layers are the same as would be separated 
out by centrifuging, or not, but this is not at all unlikely. As 
many of the species of mussles in which we have seen this con- 
dition, for example Quadrula ebena, Q. trigona, and Pleurobema 
sesopus, have brightly colored red or pink eggs, the stratification 
is quite striking, the pigments being always at the heavier pole, as 
it is invariably directed towards the lower border of the gill. 
STRUCTURE OF THE GLOCHIDIUM 
As has long been known, two well marked types of glochidia 
are found in the Unionidse, one provided with stout hooks on the 
ventral margin of the valves, the other in quite different shape and 
entirely bookless. The first is characteristically parasitic on the 
fins and the other external parts from which scales are absent, the 
second upon the gill-filaments. The occurrence of the two types 
in the genera which we have examined is shown in the following 
list: 
HOOKI.ESS GLOCHIDIA 
Lampsilis 
Obliquaria 
Obovaria 
Plagiola 
Pleurobema 
Quadrula 
Tritogonia 
Unio 
In addition to these, there is the peculiar larva of Lampsilis 
(Proptera) alatus, which has been called '^axe-head" glochid- 
ium (fig. 25). This possesses hooks which are not homologous 
with those of the anodonta type and is to be regarded as more 
nearly related to the bookless forms, an interpretation which is 
HOOKED GLOCHIDIA 
Anodonta 
Strophitus 
Symphynota 
