104 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
nucleoli. In slightly larger cells there are one or more 
clear spaces, like vacuoles, but really accumulations of 
non-staining yolk material, and the nucleus is still placed 
centrally. In larger cells still, the number of these spaces 
increases, and the cell substance diminishes relatively to 
the size of the cell. The nucleus is still central, and the 
yolk masses are placed symmetrically round it. The 
largest yolk cells are irregular in shape, and have the yolk 
substance still arranged in distinct rounded masses 
separated by narrow septa of undifferentiated cell proto- 
plasm. Finally, the cell membrane ruptures, and the 
yolk granules become free in the lumina of the vitelline 
follicles or ducts. These granules appear to be flattened, 
egg-shaped bodies, relatively large in size. They do not 
take ordinary stains, but have a yellow-brown colour of 
their own. 
The Excretory Vesicles. 
These are two prominent vesicles, one on either side 
of the pharynx. They are roughly oval in shape; open by 
fine ducts on the lateral or ventral part of the body; and 
have an epithelium, cuticular, or at least structureless in 
nature. I have been able to trace the most proximal part 
of the longitudinal excretory canals, but not the entire 
length of these vessels. 
Systematic Position. 
There is great difficulty in referring these animals to 
any known genus of Ectoparasitic Trematodes. Obviously 
they are to be included among the Tristomeae, but they 
are not Tristomidae, for the absence of the paired acces- 
sory anterior suckers, or the replacing membrane, coupled 
with the absence of radial ridges, and chitinous hooks on 
the posterior sucker certainly places them outside this 
family. The only family to which they can belong is the 
