REPORT ON THE PYCNOGONIDA. 109 
not quite separated pairs of ganglia. Finally, Dohrn states in the same paper that 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 ganglia. Of 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 Pycnogonids, I made 
use of the following specimens from the material of the Challenger Expedition :—one of 
Nymphon hamatum, one of Nymphon brachyrhynchus, two of Nymphon brevicaudatum, 
Miers; numerous specimens of Nymphon robustum, Bell; one of Colossendeis leptor- 
hynchus, one of Colossendeis megalonyx, and one of Colossendeis proboscidea, Sab. ; 
finally, one of Phowichilidium 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 far I have been successful in this may be judged from the 
following :— 
The nervous system consists in all species of Pycnogonids of a supra-cesophageal 
ganglion, an oesophageal commissure, and five (seldom four) thoracic ganglia. The 
supra-cesophageal ganglion is situated in the cephalothoracic segment ; however, its place 
varies greatly with the form of this segment, and therefore it even shows small differences 
in the different species of one genus. The differences in the different genera are more 
considerable. In the genus Nymphon 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 leptorhynchus, e.g.) from the 
remaining part of the segment by means of a constriction; the oculiferous tubercle 
is here exactly above the ganglion. In Phowichilidiwm 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 cesophageal commissures are 
very short, run parallel to each other, and enclose a narrow canal through which only 
the cesophagus passes (Pl. XVIII. fig. 11 C.), in Colossendets (I observed it in Colossendeis 
leptorhynchus, Hoek, and in Colossendets proboscidea, Sab., sp.) this canal is very wide 
(Pl. XVIII. fig. 4), the commissures which connect the supra-cesophageal ganglion with 
