REPORT ON THE PYCNOGONIDA, 



121 



"What we learn from this Hst is that it is a common feature for the shallow- water 

 species to have four distinct eyes ; and for those inhabiting a depth exceeding 400 fathoms 

 to have no eyes, or to have rudimentary eyes without pigment. While exceptions to 

 this rule are rare in the shallow-water species, they are by no means unfrequent in the 

 deep-sea species. As to the structure of those eyes which may be called rudimentary, 

 they often have a distinct lens — a rounded spot marked by its brightness ; they are quite 

 destitute of pigment, and, as I learned from a section of the oculiferous tubercle of 

 Nymphon rohustum, Bell, the small eye has no retina, and is filled up with a mass of 

 detached connective tissue. 



In those species which are furnished with distinct eyes, the size of those on the same 

 tubercle is not always the same. In Nymphon stromii, Kroyer, the eyes which are 

 directed forwards are slightly larger than those which are directed backwards, iDut this 

 difference is much more marked in the species of Phoxichilidium. This difference in size 

 — as is generally known — is a common occurrence. Grenacher^ has shown in the simple 

 eyes of the Arachnida and also of the larvae of many insects, that this difference in size 

 is often accompanied by extremely interesting differences in the minute structure. It 



' Qrenacher, H. — Untersuchungen iiber das Sehorgan der Arthropoden, 4o, Gottingen, 1879. 



(ZOOL. CHALL. EXP. PART X. 1881.) K IC 



