374 PROCEEDINGS OF THE ACADEMY OF [April^ 



and the whole cephahc nervous system is much concentrated. Behind 

 the pedal ganglia and somewhat higher dorsally may be distinguished, 

 particularly in older larvae, 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 gangha 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 larvae 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. 



Excretory Organs. 



The large excretory cell which lies 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 sHght 

 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- 



