THE NERVOUS SYSTEM OF PYCNOGOXIDS 



WILLIAM A. HILTON 

 Zoological Laboratory of Pomona College, Clarciuont, Cal. 



TWENTY-ONE FIGURES 



The noi'vou.s system of pycnosoiiids presents many ])eculiari- 

 ties. It is rather difficult to find the counterpart of this sys- 

 tem in other arthro])ods. The nervous system of some Crus- 

 tacea suggests it, especiallj' in those forms with an elongatetl 

 thoracic region and refhiced alxlomen. The general arrange- 

 ment of the gangha is totally unlike the central nervous system 

 of arachnids although the general form of the body of 'sea spiders' 

 strongly suggests arachnid relationships. The rather small 

 supraesoi)hageal ganglion and the well develoi)od chain of ven- 

 tral ganglia suggest a rather iirimitive type of nervous systrni. 

 but the imier\ation of the i^harynx and jiroboscis presents com- 

 plex and apparently unicjue conditions. 



Although there is an extensive.literature on the classilicaiiou, 

 structine and development of pj'cnogonids, there is littU^ or 

 nothing on the structure of the nervous system. 



The general form of the ganglia with their chief branches 

 is quite well known, for nearly every pajier on the classifica- 

 tion of the groui) contains a more or less detailed sketch of the 

 animals described with the nervous system shown in place. 



The supraesophageal ganglion seems to contain but two i)airs 

 of ganglia recognized by early authors in other arthropods as 

 the i^rotocerebriun and deutocerebrum, the tritocerebrum found 

 in some arthropods being absent. This is but one of several 

 structures that point to a closer relationship with arachnids 

 than with Crustacea. However, without going into further 

 reasons at this time, I am inclined to side with Dohrn and con- 

 sider Pycnogonida a separate class. 



4G3 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 26, NO. 5 

 OCTOBER, 1916 



