472 WILLIAM A, HILTON 



of this stage there seem to be two small pairs of ganglia. The 

 dorsal ganglia are not shown in figure 17. 



Wlien the larvae moult again and leave the cavity of the 

 hydroids they have all but one pair of legs. Figure 18 shows 

 the whole central nervous system from below at such a stage. 

 The brain above the esophagus is at the upper end of the figure, 

 then follow the ventral ganglia, seven paired masses and a small 

 unpaired caudal ganglion. There is a gradual fusion of these 

 ganglia until the adult condition shown in figure 7 is attained. 



The structure of the adult nervous system of pycnogonids 

 is quite simple. There is the same general arrangement of 

 cells that we find in other arthropods. The ventral ganglia 

 have few cells on the dorsal side, but many on the lateral and 

 ventral sides. The supraesophageal ganglion is sheathed in 

 cells on the lateral and dorsal sides. Nerve fibers connect the 

 ganglia and certain regions but in no place is there a concen- 

 tration of the fibers. The fibrous mass is not particularly 

 dense at any point. There do not seem to be many long tracts 

 and the supraesophageal ganglion is not more complicated 

 than other parts so far as could be determined. There are 

 no marked decussations of nerve fibers and the nerve cells pre- 

 sent a uniform appearance. Among the nerve cells are man}' 

 nuclei of neuroglia networks which form the framework of the 

 ganglia especially in the area of the cells. 



Although there are indications of special groups of cells and 

 fibers, there was no indications of mushroom bodies. 



The animals do not seem to have a special brain. The su- 

 praesophageal ganglion is not a very special center. The move- 

 ments of the animals agree with this; they move sideways, 

 forwards or backwards when stimulated. No part of the body 

 seems to lead in the locomotion. 



