THE PYCNOGONIDS. 63 



time, and these have the ectoblast very slightly thicker at the 

 edge of the eye. Possibly these may represent rudimentary 

 eyes which were placed on each side of the first pair, or else 

 have shifted position from some other place ; at any rate, they 

 are of interest and very constant in position, and furnish very 

 little ground for speculation. 



The structure of the eye of the Pantopod-larva was described 

 in Part I. This description is correct, I believe, as far as it 

 goes, but at the time I had little knowledge of the adult eye, and 

 hence overlooked an extremely narrow layer of ectoblast between 

 the two large cells and the chitin. I have since re-examined the 

 eye of this larva, and can assert that in some sections this layer 

 could be made out, and in others a thin membrane-like body 

 separated the clear cells (middle layer) from the chitin, and this, 

 presumably, is the same ectoblast. Unfortunately, the section 

 from which the figure of Plate I was made had only a very 

 narrow ectoderm layer, and was overlooked. We may affirm, 

 then, that the eye at all stages of development is a three-layered 

 structure. 



Comparison with the Arachnid Simple Eyes. 



We are now in a position to make, if possible, a practical 

 application of the preceding. Does a study of the structure and 

 development of the Pycnogonid eye throw any light upon the 

 relationship of the Sea-Spiders to other groups % I think there 

 can be no doubt of this, and that it furnishes a most satisfactory 

 verification of the relationship pointed out in the preceding 

 sections. 



It seems absolutely impossible to bring the eyes into the same 

 category with the eyes of the Annelids, which is a decided gain 

 for our position, and it is par excellence an Arthropod eye, and 

 even more, as I hope to show it must be referred to a particular 

 group of Arthropods — the Arachnids. The absence of any very 

 close resemblance to the eyes of Annelids would not be in itself- 

 a final argument against Dohrn's position, inasmuch as the eyes 

 may have been much modified during the transition from 

 Annelid to Pycnogonid ; but this can scarcely be the case, for 

 the eyes seem to show traces of degeneration in many, if not in 

 all (as shown, perhaps most strikingly, by the large vacuoles, and 



