83 



there are collections of nerve cells along certain nerves, 

 in particular the circumpallial nerve (fig. 26, N. c), 

 which renders the latter almost a fourth ganglion, both 

 structurally and functionally. 



Cerebral ganglia. The pair of ganglia known as the 

 cerebral, or better — the cerebro-pleural, representing the 

 fused cerebral and pleural ganglia occurring in Nucula 

 and Solenomya, are found best by pulling the lower lip 

 dorsally and moving the labial palps aside, so as to lay 

 bare the area between the lower lip and the foot. The 

 ganglia will then be seen faintly through the overlying 

 tissue, which must be removed carefully or they will be 

 pulled away with it. They are situated a considerable 

 distance below the mouth and oesophagus, and very close 

 to the pedal ganglia. Each ganglion (fig. 27. G. ch.) 

 shows indications of being bilobed, and this is further 

 borne out by sections wdiich show a distinct, though not a 

 deep, division into two lobes. From the upper corner 

 arises the cerebral commissure (figs. 2G and 27, Com.). 

 This connects the two ganglia and passes dorsally over 

 the oesophagus, but owing to the position of the cerebro- 

 pedal ganglia it is a larger loop than is usually the case. 

 From the outer sides of the ganglia, just below the 

 depression dividing them, arise the anterior pallial nerves 

 (figs. 27 and 26, N . pa.), and a smaller nerve which gives 

 branches to the labial palps (fig. 27, N . I.). 



The Anterior Pallial Nerve lies close to the cerebral 

 commissure, and passes with it to the side of the 

 oesophagus, embedded m the digestive gland. It rises to 

 the surface and enters the mantle just dorsal to the upper 

 lip where this joins the outer labial palp on each side, 

 then, dividing into two branches, it passes to the margin 

 of the mantle and joins the circumpallial nerve (fig. 26, 

 N. c.) by several branches. 



