74 DR. H. J. HANSEN ON CRUSTACEANS [Jan. 20, 
on the lower surface (fig. 27) four organs, one of which is large 
and two of the others very small; on the third and fourth 
segments these organs are gradually reduced in number, and none 
of them are found on the fifth. The sixth abdominal segment 
has a median row of six organs, which are seen in fig. 24 with 
the exception of the first, this being hidden by the lateral wings 
of the preceding segment. The very short basal jomt of the 
peduncle of each of the pleopods has on the outer side a small 
organ, looking outwards and at least sometimes a little down- 
wards. Hach uropod has one organ on the peduncle, situated on 
its inner surface near the lower margin and close to the base, 
besides one organ on the interior (lower) surface of the inner 
branch near its base; finally, two organs on the interior (lower) 
surface of the outer branch, one of them near the middle and the 
other on the distal narrowing part. 
From the preceding description it is seen that most of the organs 
look downwards, a smaller number somewhat outwards or forwards, 
rather few almost totally outwards, and none upwards, with 
the exception of one on the first maxilliped. With the exception 
of the few lateral organs on the scutum, all the others are found 
on or near the lower surface of the body and on the appendages. 
The structure of the organs is very interesting, and very 
different from all hitherto known in any invertebrate animal. I 
have examined more closely three organs, viz., that situated on 
the third joint of the first maxilliped, one of the “stalked” 
organs near br. above trl.*, and one from the antero-inferior 
margin of the fourth abdominal segment. The two last-named 
organs have been cut off, most of the adhering tissue removed 
by dissection, and the organs examined with moderately high 
magnifying power. I have found no difference of any importance 
between the three organs from such distant parts of the animal’s 
body. 
The organ taken as type is that from the infero-lateral margin 
of the thorax; it has been drawn (fig. 2 m) in optical vertical 
section. The skin forms a chitinous, large, and very thick biconvex 
lens (a), which is vitreous and a little yellowish; the major 
central part is covered by a rather thin limpid layer (a’), but this 
layer I could not perceive on the two other organs examined. 
The lens is circular in outline; its diameter is about two-thirds 
as long as that of the inner portion of the organ at its thickest 
part. “The inner side of the biconvex lens is covered by a large 
and rather thick coneavo-convex lens (6), which is sormewlant 
thinner than the outer lens, but with the diameter a little longer 
than that of the same; the lateral margin of the inner lens is 
oblique, very broad, touching the external chitin around the 
outer lens. This inner lens, which consists of two layers, is 
homogeneous, vitreous, and slightly greyish ; but the difference 
between the colour of the outer and the inner lens is in the figure 
purposely a little more strongly marked than in nature. These 
two lenses remind one of optical struments in which the lens is 
