494 
THE VOYAGE OF H.M.S. CHALLEHGEK. 
last segment behind the anterior region, are split by a median groove. The second last 
scute just mentioned is separated into two nearly equal halves, the left being only a 
shade larger. The furrow then enters the sulcus between the segments, and, lea\dng 
the scutes, passes obliquely forward to the right over the ventral aspect of the first 
segment of the posterior region to the groove between it and the last thoracic, and 
continuing upward, forms an oblique and very perceptible furrow on the right half of 
the last dorsal segment. A very slight groove runs forward to the cephalic region, but 
as it is broad and shallow its independent existence is not distinct. 
The cephalic region presents dorsally, on each side of the median line, a kind of flap, 
which is connected by a thin membrane with the prominent lateral edge. The collar 
runs downward without break to the ventral surface, and terminates in a prominent and 
somewhat thin triangular flap on each side of the median fissure. 
The branchiae number about twenty-four on each side, and the basal web is short. 
Except the first dorsal, which is tinted brownish, the radioles are pale. The first 
dorsal is the strongest radiole on each side, and it has, toward the tip, a very large eye, 
which is entire internally, but externally is split by the axis passing to the bare ter- 
minal process beyond it. The latter is both shorter, deeper, and more flattened than in 
any of the other branchial radioles. The surface of the greatly enlarged eye is minutely 
dotted, as if furnished with minute corneal facets analogous to those of Crustaceans 
and Insects, and it has proximally a kind of peduncle or process. The other radioles 
are more slender, but have a similarly arranged though much smaller eye in each case, 
and the terminal process is long and tapering. The eyes seem to be absent on each 
side in the two ventral radioles, which, like the adjacent organs, are somewhat shorter 
than the others. The precise nature of the arrangement in this respect, however, is 
open to further investigation, since the majority have been mutilated and their tips in 
process of reproduction. The pinnae are brownish, and here and there white, and they 
do not diminish much at the tip. 
The tentacle is comparatively short and pale, the base being broad and frilled and 
the tip slender. At the ventral edge of each branchial fan is a hood-like membranous 
fold, which in the contracted state of the parts opens ventrally. 
The thoracic or anterior region consists of nine segments. The bristles form two 
sets, a long dorsal series (PI. XXXa. fig. 10) with slender tapering tips and very narrow 
wings. The inferior bristles have shorter extremities and broader wings (PI. XXXa. 
fig. 11), and, moreover, they seem to be minutely spinous. Both kinds possess a 
peculiar stiff appearance. Posteriorly the wings in both sets are better developed, and, 
as usual, the tips are much elongated. 
The anterior hooks (PI. XXXa. fig. 12) have a prominent though rather small crown, 
which is minutely denticulated. The dorsal line below the crown is nearly straight, and 
the basal process is greatly elongated. The anterior projection or prow is moderately 
