4()4 WILLIAM A. HILTON 



As the tendency has been to regard these animals as arachnids, 

 it may be worth while to glance through the neurological litera- 

 tiu-e on this group. 



Among the earliest work on the nervous system of arachnids 

 was that of Treviranus in 1812. No hint of pycnogonids is 

 given in this paper, nor is there any mention of these animals 

 in the work of B. Haller just a century later. There is no refer- 

 ence to pycnogonids in the extensive work of Saint Remy, '90. 

 Dahl, in 1913, gives a brief summary of the work of Dohrn 

 in connection with various types of arachnids. If we go through 

 the extensive literature on the pycnogonids as a group we find, 

 it is true, little of the structure of the nervous system, but much 

 about the arrangement of the ganglia composing it. 



From' the works of Hoek, 78, '81, Dohrn '70, '81, Sars '91, 

 Meinert '98, and a number of others, as well as from the study 

 of Pacific coast forms, we learn that the central nervous sys- 

 tem consists of a supraesophageal ganglion and a ventral chain 

 of from four to five chief ganglia. The smaller number of gan- 

 glia we find when the body is less elongate. The supraesopha- 

 geal ganglion has a ventral median nerve to the proboscis, 

 nerves to the eyes and a pair to the chelifori. Each ventral 

 ganglion has at least one main branch. Three branches from 

 the first ventral ganglion are as follows: 1) A small pair or two 

 pairs to the proboscis; 2) a pair to the palps; 3) a pair to the 

 ovigers; 4) if the first ganglion is fused with the second as it is 

 in those with four ganglia, then there is also a pair to the first 

 pair of walking legs. 



Figures 1 to 7 show different types of nervous systems from 

 Pacific species of pycnogonids. The method by which the 

 nervous system was studied by some observers was simply to 

 determine the position of the ganglia through the transparent 

 body-wall. This was tried with a number of specimens after 

 the animals had been fixed in mercuric fluids. In some cases 

 the whole animal was stained and mounted in such a way as 

 to show the internal ganglia. In some cases the animals to 

 be studied were placed for a short time in caustic or acid and 

 by one or the other of these methods the internal parts were 



