122 THE VOYAGE OF H.M.S. CHALLENGER. 
would be of interest to study this question in the Pyenogonida, but the limited number of 
the specimens of the different species of Phowichilidiwm in the Challenger collection did 
not allow me to study their eyes. I investigated those of Nymphon strémi, but there 
the difference in size is slight ; I did not ascertain any difference in their structure. 
The knowledge of the minute ‘structure of the eyes of the Pycnogonids is of very 
recent date. They were always considered as simple eyes, and were even mentioned 
as such by Cavanna. Dohrn, therefore, in the preliminary -publication on the results 
of his studies on Pyenogonids, is the first who. gives us some information on these 
organs. According to him the eyes, taken in a vertical. position, are of a pointed 
oval shape;* they have a retina composed of modified epithelium cells (hypodermic 
cells), the extremities of which are surrounded by a brown pigment; the cuticula forms 
a lens. | 
Among the latest investigations into the structure of the eyes of the Arthropoda, those 
of Grenacher, published in his splendid memoir,’ have the merit in the first place not 
only in proposing a homology between the parts composing a compound and those com- 
posing a simple eye, but also in showing the existence of this homology throughout almost 
the whole type of the Arthropods. Moreover, the value of his monograph with regard 
to the physiology of the organ of sight, and the immense quantity of special information 
which it contains, is at present almost inestimable. In investigating the eyes of the 
Pyenogonids, I was extremely desirous to try whether the scheme for the eye of the 
Arthropods, as proposed by him, would hold good also in the case of the Pycnogonids. 
Though my researches did not give me a complete idea of the anatomy of their eyes, 
I think my results are worth publishing here, because they admit of comparison with 
the scheme given by Grenacher.’ ‘aie 
I made numerous preparations of the eyes of different species of the genus Nymphon ; 
of N. brevicaudatum, Miers, N. brachyrhynchus, Hoek, N. strémi, Kroyer, and of 
the rudimentary eyes of Nynyphon robustwm, Bell. I studied more. especially those I 
made of Nymphon brachyrhynchus and of Nymphon strémi. While the eyes of Nymphon 
strénai (Pl. XVIIL. fig. 11, B) are comparatively small, and placed on the sides of a conical 
tubercle, so as to be separated by a greater distance from one another below than above ; 
those of Nymphon brachyrhynchus are larger, and are placed on the inside of a cylindrical 
oculiferous tubercle, so close to one another as to meet at their inner surfaces. In fig. 2, 
1 Von oben nach unten spitz oval (loc. cit., p. 37). 
* Grenacher, H.—Untersuchungen uber das Sehorgan der Arthropoden, 4°, Gottingen, 1879. ° 
8 For studying the histology of the Arthropod’s eye fresh material, in the’ first place, is necessary. That I 
got preparations fit to be used of the eyes of Nymphon brachyrhynchus, collected at Kerguelen Island, and put in 
spirits in January 1874, is almost more than could be expected. In one respect only did the condition of the material 
hinder me, viz., that I could not make use of some reagents, ¢.g., of nitric acid. Though used of different strengths, I 
never obtained the solution of the pigment without the visual rods being destroyed also. Consequently I never got 
a preparation showing the connection between the fibres of the optic nerve and these rods. 
