372 OILMAN A. DREW. 



dorsal space. This dorsal movement includes the cerebi-al 

 ganglia (eg.). When casting- is completed^ and the valves of 

 the shell are closed, a further dorsal movement occurs, that 

 results in the fiUiug of the greater part of the dorsal space. 

 This movement places the cerebral ganglia in position poste- 

 rior to the anterior adductor muscle (figs. 35 and 36, eg.). 

 The foot now begins to grow quite rapidly, and the pedal and 

 visceral ganglia begin to form (fig. o6, pg. and vg.). Both 

 pairs of these ganglia are formed as thickenings of the 

 surface ectoderm. The thickenings that give rise to the 

 pedal ganglia begin to form first, but both pairs of ganglia 

 are in process of formation at the same time. Owing to the 

 character of the embryonic tissue it is very difficult to 

 determine how the commissures that connect the ganglia 

 arise. They are first found very close to the surface, almost, 

 if not quite, in contact with the ectoderm. Later they sink 

 deeper into the body. The cerebro-visceral commissures are 

 quite thick, and differ from the cerebro-pedal commissures in 

 having much the same structure as the ganglia themselves. 

 In the earlier stages I have been able to demonstrate only a 

 single cerebral origin for each cerebro-pedal commissure. 

 This may be due to the difficulty of tracing commissures in 

 embryonic tissue. Later stages show two separate origins 

 very distinctly. 



The double origin of the cerebro-pedal commissures has 

 been regarded by Pelseneer (13) as an indication of the 

 presence of cerebral and pleural ganglia in each anterior 

 nerve-mass. Furthermore, Pelseneer and others find that 

 each mass is divided by a constriction into two rather distinct 

 parts. I have not been able to satisfy myself that there is a 

 distinct separation into cerebral and pleural gangliaj either 

 in this or the other forms that I have studied. 



The cerebral and pedal ganglia are about equal in size, but 

 they differ in shape (fig. 48). The visceral ganglia are 

 smaller than the cerebral ganglia, but compare pretty well 

 with them in shape. Each cerebral ganglion is large at its 

 anterior end^ and tapers posteriorly into the cerebro-visceral 



