122 THE VOYAGE OF H.M.S. CHALLENGER. 



would be of interest to study this question in the Pycnogonida, but the hmited number of 

 the specimens of the different species of Phoxichilidium in the Challenger collection did 

 not allow me to study their eyes. I investigated those of Nymphon stromii, l:)ut there 

 the difference in size is slight ; I did not ascertain any difference in theii- 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 Pycnogonids, 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 memoii",^ 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 

 Pycnogonids, I was extremely desirous to try whether the scheme for the eye of the 

 Ai-thropods, 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 theii" eyes, 

 I think my results are worth publishing here, because they admit of comparison with 

 the scheme given by Grenacher.^ 



I made numerous preparations of the eyes of different species of the genus Nymphon ; 

 of N. brevicaudatitm, Miers, N. brachyrhynchus, Hoek, N. stromii, Kroyer, and of 

 the rudimentary eyes of Nymphon robustum, Bell. I studied more especially those I 

 made of Nymphon brachyrhynchus and of Nymphon stromii. While the eyes of Nymp)hon 

 stromii (PI. XVIII. 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 cyLLudrical 

 oculiferous tubercle, so close to one another as to meet at their inner sm-faces. In fig. 2, 



' Von oben nach unten spitz oval {loc. cit., p. 37). 



- Grenacher, H. — Untersuchungen uber das Sehorgan der Arthropoden, 4°, Gottingen, 1879. 



' For studying tlie 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 ia74, is almost more -than could be expected. In one respect only did the concUtion of the material 

 hinder me, viz., that I could not make iise of some reagents, e.g., of nitric acid. Thongh used of different strengths, I 

 never obtained the solution of the pigment ^vithout the visual rods being destroyed also. Consequently I never got 

 a preparation sho-^ving the connection between the fibres of the optic nerve and these rods. 



