374 PROCEEDINGS OF THE ACADEMY OF [April, 
and the whole cephalic nervous system is much concentrated. Behind 
the pedal ganglia and somewhat higher dorsally may be distinguished, 
particularly in older larve, the rudiments of the pleural ganglia, which 
also appear to have arisen by delamination of the ectoderm and lie in 
close association with cerebral and pedal ganglia. A very heavy 
commissural strand connects the two pedal ganglia, and the whole 
nervous system of the larva foreshadows in its compact structure the 
adult condition, individual ganglia being difficult to distinguish. Figs. 
92 and 94 show sections through this region at a somewhat later 
period than figs. 88 and 89. Eyes have not developed to a functional 
condition in the oldest larve observed. Sections of these show pig- 
ment granules within cells lying close to the cerebral ganglia, and in 
some cases these cells lie around a slight invagination of the ectoderm— 
the first evidence of optic organs. 
a 
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Excretory Organs. 
The large excretory cell which hes on the right side of the larva and 
forms the chief member of a group of similar greatly vacuolated cells 
lying in that region arises from the third quartet in the C quadrant, 
and from its large size and conspicuous appearance its complete history 
is known. Returning to a segmentation stage, in which the egg con- 
tains about one hundred and twenty cells (fig. 70), it will be seen that 
the third quartet group in C quadrant contains seven cells. Divisions 
next occur in the three large cells, 3c”, 3c" and 3c”" (fig. 77). The 
cell 3c! does not divide with these, nor does it ever again divide, but 
continues its growth, soon becoming the largest element in the ecto- 
derm. As gastrulation proceeds this large cell, 3c (Ex.), the origin 
of which is thus established, appears at the right of the elongating 
gastrula (left of figs. 78,,79) and with the closure of the blastopore lies 
midway between dorsal and ventral surfaces, as shown in figs. 98 and 
99. It has become much larger, when compared with its neighboring 
cells. both from lack of division and by actual growth. As the veliger 
takes form this cell becomes’ yet more marked (fig. 102), and when 
the shell gland has become prominent (fig. 104) it is seen lying in a slight 
depression surrounded by small cells which are in an active state of 
division. As the foot arises and the cephalic end of the veliger is 
differentiated from the body, the large excretory cells move upward 
along the body just posterior to the pedal groove, on the right side, 
this change of position being a natural sequence of the general torsion 
of that region (figs. 105, 106). The intestine has also become well 
developed by this time as a solid strand of cells connecting the pos- 
