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 

 eao-erly studied is, without doubt, the nervous system, and this is tjuite natural, because 

 it has been rightly considered, that if any system could be expected to shed Hght 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 Nijnvplion is not a very happy one, as he descril)es and figures it as consist- 

 ing of a supra-oesophageal 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-oesophageal 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 PaUene and in Achelia, 

 on the contrary only four were observed by him in three species of Fhoxichilnlhim. In 

 my paper the optic nerves of Pycnogonum are described, and the numljer of gangha in 

 Nymjyhon is given as five, in Pycnogonum as four.^ We find in Dohrn's latest paper {loc. 

 cit., p. 37) a much more detailed description of the structui^e of this system. The supra- 

 oesophageal ganglion innervates the mandibles, and, moreover, gives ofi" 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 ofi' three pairs of nerves ; 

 the first pair arising from the ganglion a little outside and below the insertion of the 

 circum-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 prolwscis, and these three ganglia are connected by commissures, which 

 thus form a secondary oesophageal 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 " fibrilliiren 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 Sucli a long appendage, at the tip of which the gland opens, occurs also in Ammothea (Dohrn, loc. cit., p. 36). 



2 Semper (C), tjber Pycnogonideu und ihre in Hydroideu schmarotzenden Larvenformen (Arljeiten a. d. Zool.- 

 Zoot. Institut in Wiirzhurg, i., 1874, p. 278). 



3 Loc. cit., p. 249. 



