REPORT ON THE PYCNOGONIDA. 121 
nT a A 
Depth Number, 
Name of Species. in - 
Fathoms, of Eyes. 
Condition of the Eyes. Observations. 
| Nymphon hamatum, Hoek, . - | 1375-1600 Soc aes Oculiferous tubercle also 
we rudimentary. 
Colossendeis gigas, Hoek, . b . | 1875-1600 oa ore Oculiferous tubercle of the 
youngerspecimens higher, 
and furnished with two 
rudimentary spots. 
Colossendeis gracilis, Hoek, . . | 1375-1600 =a Oo, Oculiferous tubercle much 
; ; elevated, with four lenses. 
Phoxichilidium pilosum, Hoek, . | 1600-1950 | Four, | Two large ones directed forward, 
two very small ones backward; 
: tubercle elevated and acute. 
Nymphon meridionale, Hoek, . len LGD) - Distinctly pigmented ; tubercle not 
k very high. 
Phowichilidium oscitans, Hoek, : 1675 5 Two very large ones and two small 
. : ones. 
Phoaichilidium mollissimum, Hoek, 1875 5 Two extremely large ones, kidney- 
shaped, directed forward; two 
very small ones directed back- 
ward. 
Nymphon procerum, Hoek, . 5 2160 doc : 600 ‘Two small spots are present 
; on the tip of a low 
ee : tubercle. 
Nymphon longicollum, Hoek, . 5 2225 ane me - | Oculiferous tubercle high, 
NA with two rudimentary 
: spots without pigment. 
Colossendéis media, Hoek, - . : 2225 Boo ee Oculiferous tubercle very 
high and acute. 
Colossendeis brevipes, Hoek, . : 2650 aes op Oculiferous tubercle high 
: and acutely pointed. 
What we learn from this list 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 robustum, 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 Mymphon strémii, Kroyer, the eyes which are 
directed forwards are slightly larger than those which are directed backwards, but this 
difference is much more marked in the species of Phowichilidium. 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 larvee of many insects, that this difference in size 
is often accompanied by extremely interesting differences in the minute structure. It 
1 Grenacher, H.—Untersuchungen tiber das Sehorgan der Arthropoden, 40, Gottingen, 1879. 
(ZOOL. CHALL. EXP,—PART x,—1881.) - K 16 
