1904. ] NATURAL SCIENCES OF PHILADELPHIA. 375 
terior end of the enteric cavity with the ectoderm, and this latter point 
of contact is Just below the large excretory cell. Fig. 88 shows a sec- 
tion through this region, showing the excretory cell to be much vacuo- 
lated and to lie for the most part below the ectoderm. At a consider- 
ably later stage (figs. 109, 110) its position and structure are shown 
just before the veliger escapes from the egg capsule. A large nucleus, 
which usually contains several small nucleoli and having the general 
appearance of nuclei in cells which have for a long time remained 
undivided, lies at the lower end of the cell. The cytoplasm is greatly 
vacuolated and at its peripheral end, where it meets the exterior, is 
seen a deep pit with constricted mouth. This appears to function 
as an intra-cellular duct, for it comes into connection at its inner end 
with the large vacuoles which fill the cell. Just above and anterior 
to the large cell is a group of smaller ones which contain darkly stained 
nuclei and pigment granules. One of these, the largest, also contains 
vacuoles and lies nearest the cell 3c". In somewhat older larve one 
or two of these smaller cells, which lie close to 3c", have increased 
much in size, become greatly vacuolated and appear to function as 
their larger neighboring cell. These smaller accessory excretory cells 
are also doubtless of ectodermal origin and, since they lie between the 
principal one and the blastopore, are doubtless derived from the same 
quartet. 
In addition to the excretory cells above described others of a similar 
nature are found in the larva of Fiona. Sections (figs. 90, 91) of fairly 
well-developed veligers show two cells (Nph) nearly symmetrically 
placed on the two sides of the body just behind the constriction sepa- 
rating head from body region. These cells contain large nuclei and 
their protoplasm is clear and greatly vacuolated. In a slightly older 
stage (the oldest larvae examined) yellowish-brown granules are very 
evident, lying in the meshwork of the vacuolated cytoplasm. The 
cell on the left side (fig. 91) lies just to the side of and slightly higher 
than the otocyst of that side, being closely associated with its ganglia, 
while the one on the right side (fig. 90) lies higher and is in close prox- 
imity to the smallest cells of the large excretory organ of that side. It 
may be distinguished from the cells of this organ by its clear cytoplasm 
and the color of the granules lying in it. In later stages another cell 
of similar nature may be seen beside the one on the right side, but 
only one has been observed on the left. The origin of these cells is not 
known. In earlier stages cells of slightly smaller size lie in the regions 
which they later occupy, but cannot be distinguished in structure 
from neighboring mesodermal elements. However they lie close to 
