108 THE VOYAGE OF H.M.S. CHALLENGER. 
a distinct swelling (a kind of receptaculum) near the beginning.’ About the structure 
of the gland itself in this species I have no observations to communicate. 
2. Nervous System.—Of the different systems of the Pycnogonida the one most 
eagerly studied is, without doubt, the nervous system, and this is quite natural, because 
it has been rightly considered, that if any system could be expected to shed light on 
the affinities of the Pycnogonids with the other Arthropoda, it would be the nervous 
system. 
Among the more important papers on the subject, those of Zenker, Semper, Dohrn, and 
myself may be mentioned. The way in which Zenker (loc. cit.) treats of the nervous 
system of Nymphon is not a very happy one, as he describes and figures it as consist- 
ing of a supra-cesophageal ganglion and four thoracic ganglia. The account given by 
Semper?” of the nervous system of this genus is much more accurate. He tells us that in 
Nymphon the supra-cesophageal ganglion innervates the mandibles and the eyes, and that 
the first of the five thoracic ganglia furnishes nerves to the proboscis, to the palpi, and 
to the ovigerous legs, while the four following ganglia give off nerves to the four legs. 
The number of thoracic ganglia is, according to Semper, also five in Pallene and in Achelha, 
on the contrary only four were observed by him in three species of Phowichilidium. In 
my paper the optic nerves of Pycnogonwm are described, and the number of ganglia in 
Nymphon is given as five, in Pycnogonum as four.’ We find in Dohrn’s latest paper (Joc. 
cit., p. 87) a much more detailed description of the structure of this system. The supra- 
cesophageal ganglion innervates the mandibles, and, moreover, gives off an azygous nerve, 
which dorsally innervates the proboscis, and forms a ganglion at about one-third from 
the extremity of the proboscis. The first thoracic ganglion gives off three pairs of nerves ; 
the first pair arising from the ganglion a little outside and below the insertion of the 
cireum-cesophageal commissures, innervates the lateral parts of the proboscis. Like 
the azygous proboscideal nerve, they form ganglia at about one-third from the 
extremity of the proboscis, and these three ganglia are connected by commissures, which 
thus form a secondary cesophageal ring. The second pair innervates the so-called palpi; the 
third arises from the ganglion laterally towards the posterior part, it innervates the ovigerous 
legs. Moreover, Dohrn observed that this first thoracic ganglion not only in the genera 
furnished with palpi and ovigerous legs, but also in those forms which have lost their 
palpi and even in the females, which have lost also their ovigerous legs, consists of three 
nuclei of “ fibrilliren Punktmasse,” each of which gives off the fibres for the nerves re- 
spectively of the proboscis, palpi and ovigerous legs. In a young stage of the embryological 
development, Dohrn made the observation that the first ganglion really consisted of two 
1 Such a long appendage, at the tip of which the gland opens, occurs also in Ammothea (Dohrn, loc. cit., p. 36). 
2 Semper (C.), Uber Pyenogoniden und ihre in Hydroiden schmarotzenden Larvenformen (Arbeiten a. d. Zool.- 
Zoot. Institut in Wirzburg, i., 1874, p. 278). 
3 Loc. cit., p. 249. 
