REPORT ON THE PYCNOGONIDA. 109 



not quite separated pairs of ganglia. Finally, Dohrn states in the same paper tliat besides 

 the five (six) double ventral ganglia there are two others, which, however, in some 

 genera, totally disappear, and in other genera are preserved only in a rudimentary con- 

 dition. Accordingly, Dohrn observed immature stages of Phoxichilus, in which, behind 

 the sixth ventral ganglion, were present two distinctly separated, although much smaller, 

 pairs of long ovate gangHa. C)f these the first pair gives off no nerve, and the second pair 

 the two nerves for the abdomen. 



For my observations with regard to the nervous system of the Pycnogouids, I made 

 use of the following specimens from the material of the Challenger Expedition : — one of 

 N'tjmj)ho)i hamatum, one of Nymphon hrachyrhynchus, two of Nymphon hr^vicaudatum, 

 Miers ; numerous specimens of Nymphon rohnstum, Bell ; one of Colossendeis leptor- 

 hynchus, one of Coloasendeis megalonyx, and one of Colossendeis proboscidea, Sab. ; 

 finally, one of Phoxichilidium pilosum. What I tried to ascertain in my investigations 

 was, in the first place, the innervation of the cephalic appendages and of the proboscis, 

 in the second place the structure of the first thoracic ganglion, in the third place 

 that of the last thoracic ganglion, and the presence or absence of the two rudimentary 

 abdominal ganglia. In how for I have been successful in this may be judged from the 

 following : — 



The nervous system consists in all species of Pycnogonids of a supra-oesophageal 

 ganglion, an oesophageal commissure, and five (seldom four) thoracic ganglia. The 

 supra-oesophageal ganglion is situated in the cephalothoracic segment ; however, its place 

 varies greatly with the form of the segment, and therefore it even shows small difierences 

 in the different species of one genus. The differences in the difierent genera are more 

 considerable. In the genus Nympjhon it is, as a rule, placed towards the hinder 

 extremity of the cephalic part of the cephalothorax, below the oculiferous tubercle, and 

 above the insertion of the two ovigerous legs. In the genus Colossendeis this 

 ganglion is found nearly in the middle of the cephalic part of the cephalothoracic segment, 

 which part is usually separated (distinctly in Colossendeis lepAorhynchus, e.g.) from the 

 remaining part of the segment by means of a constriction ; the oculiferous tubercle 

 is here exactly above the ganglion. In Phoxichilidium the ganglion is found also 

 above the insertion of the ovigerous legs ; but the oculiferous tubercle is in most species 

 of this genus situated quite anteriorly, on that part of the cephalothoracic segment which 

 overhangs the proboscis. As a rule the shape of this ganglion is round (PI. XVIII. 

 fig. 6), but in Colossendeis the comparatively small ganglion is much broader than long 

 (PL XVIII. fig. 4). Whereas, in the other genera the two oesophageal commissures are 

 very short, run parallel to each other, and enclose a narrow canal through which only 

 the oesophagus passes (PI. XVIII. fig. 11 C), in Colossendeis (I observed it in Colossendeis 

 leptorhynehus, Hoek, and in Colossendeis prohoscidea. Sab., sp.) this canal is very wide 

 (PI. XVIII. fig. 4), the commissures which connect the supra-oesophageal ganglion with 



