276 R. T. GUNTHER, 
Their shape is rounded, and they are often provided with a 
few processes extending inwards between the other cells 
towards the cells of the first layer. Their protoplasm is ex- 
ceedingly granular, and contains one or more large vacuoles. 
The nucleus is a very large clear spherical body in the centre 
of the cell, which hardly stains at all, almost all the chromatin 
being concentrated in the large nucleolus. The other kind of 
cells differ from the ova in their protoplasm being quite clear 
and free from granules, and in their not growing to such a 
large size. From the way in which they are situated with 
regard to the ova, they would seem to be merely packing or 
interstitial cells (fig. 3, st.). 
The third or outer layer is the same in both sexes. It con- 
sists of a covering epithelium of more or less cubical cells with 
round nuclei (figs. 3 and 4, ect.). In the female the cells are 
somewhat more irregular in shape than in the male. 
B. THe ENDODERM. 
Epithelium of Gastro-vascular System.—The endo- 
dermal lining of the manubrium in the region of the sexual 
cells has been described above in the section on the repro- 
ductive organs. On either side of this region the endoderm 
cells (fig. 5, end. m.) have much the same internal granular 
structure, but they are not so long. The epithelium covering 
the dorsal wall of the stomach is represented at end. in 
fig. 5. It consists of granular columnar cells with a clear space 
towards their free ends. No traces of intracellular digestion 
were observable in any of the cells of the gastric epithelium. 
The radial and circular canals are lined with squarish poly- 
gonal cells, many of which are vacuolated. The epithelial 
lining of the circular canal on the side nearest the nettle-ring 
is thrown into folds (end., fig. 6), the cells of which are some- 
times so tightly packed as to obscure the divisions between the 
folds. In this way a highly peculiar endodermal organ is 
formed in the circular canal, which may grow to such an extent 
as to completely fill the lumen of the canal. The cells of this 
