Reprinted from The Joirnal of Comparative Neurology, Vol. 26, No. 5 

 Octobtr, 1916 



THE NERVOUS SYSTEM OF PYCNOGONIDS 



WILLIAM A. HILTON 



Zoological Laboratory of Pomona College, Claremont, Cal. 



TWENTY-ONE FIGURES 



The nervous system of pycnogonids presents many peculiari- 

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

 tem in other arthropods. The nervous system of some Crus- 

 tacea suggests it, especially in those forms with an elongated 

 thoracic region and reduced abdomen. The general arrange- 

 ment of the ganglia 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 

 supraesophageal ganglion and the well developed chain of ven- 

 tral ganglia suggest a rather primitive tj^pe of nervous system, 

 but the innervation of the pharynx and proboscis presents com- 

 plex and apparently unique conditions. 



Although there is an extensive literature on the classification, 

 structure and development of pycnogonids, there is little 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 paper on the classifica- 

 tion of the group 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 pairs 

 of ganglia recognized by early authors in other arthropods as 

 the protocerebrum 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. 



463 



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

 OCTOBER, 1916 



