No. 407.] A SINGULAR ARACHNID. 843 
pair of appendages, presumably the serial homologues of the 
cephalic and thoracic limbs. The appendages of the second 
and third segments have come together in the median line to 
form four peculiar setigerous valves surrounding the genital 
orifice lying between these segments. The shape of the 
valves and the characteristic arrangement 
of their bristles are shown in Fig. 2. The 
anterior pair, which I regard as the append- 
ages of the first segment, are arcuate and 
closely applied to each other in the mid- 
ventral line. Their posterior edges are 
raised on either side into six papilla, each 
capped with a strong bristle. The posterior 
pair are flattened and enclose the orifice of 
the chitinous genital conduit between their 
bases. Their posterior edges are fringed 
with a series of graduated bristles. <A 
profile view of the abdomen shows that 
the genital duct runs forward into the second abdominal 
segment. Thus Koenenia would seem to agree with other 
Arachnida in having the reproductive orifice on the second 
abdominal segment. 
The ventral portions of the fourth, fifth, and sixth segments 
are nearly alike in structure. Each is provided with a single 
row of powerful bristles, which, though more or less inter- 
rupted in the median line, are nevertheless far more closely 
aggregated than those on the dorsal surface of the correspond- 
ing segments. On either side near the posterior edge of the 
segment there is a flap, the exact outline of which is not so 
clear as represented in Fig. 2. In many specimens a very 
delicate sac may be found evaginated from under the flap on 
all three segments. These sacs are in all probability lung- 
books. They appear to be the only respiratory organs of 
Koenenia, apart from the delicate integument, which, in so 
small an animal, must of itself nearly suffice for respiratory 
purposes. I have had no better success than Grassi and 
Hansen and Sorensen in finding any traces of tracheæ. If I 
am correct in regarding the above-described sacs as lung-books, 
