406 
THE VOYAGE OF H.M.S. CHALLENGER. 
arch superiorly, and a median ventral plate as in Thelepus. It forms a well-marked 
ridge behind the cephalic rim dorsally, but this gradually becomes absorbed on each 
side into a flattened area behind the infra-oral lobe. The next three segments differ 
from those of Thelepus in presenting a process or lobe jutting forward from the anterior 
border on each side, and in having a long simple branchia passing in each case from 
the same border (anterior) a little dorsad of the lobe just mentioned. The branchise are 
proportionally large, massive, and long, quite as large as those of the Ampharetidse. 
Each springs by a broad base from the extreme front of the segment to which it 
belongs, and tapers to a blunt point, the entire surface being crenate, and marked by a 
ventral line, probably from the blood-vessel. They thus differ from the frilled and 
deeply (dorsally) grooved tentacles. The posterior pair are nearest the middle line of 
the dorsum, only a brief interval separating them ; the next are the first or anterior 
pair, which have a wider space between them ; while the second pair are most external 
in position, the inner border of the base being quite clear of the tliii’d pair, and only 
slightly overlapping the outer edge of the anterior pair. This condition of the 
respiratory system seems to be unique in the Terebellidse, combining, as already stated, 
the branchial characters common to another family, \iz., the Ampharetidse. The three 
segments alluded to have rudimentary bristle-tufts. Each of the next two rings bears 
a small bristle -bundle superiorly, then a rounded papilla, and inferiorly a short hook- 
row, the whole forming a lateral band. The papilla is visible in the next segment, but 
thereafter disap]3ears ; the ordinary condition of the foot being a setigerous process 
dorsally with its tuft of bristles, and then a long uncinigerous pad or ridge. 
About a dozen anterior segments show ventral scutes, the glandular tissue of which 
is not circumscribed as in Thelepus, but passes upward at each side, and thus gives a 
character to the region. 
Twenty pairs of bristles are visible behind the three branchial segments. The 
bristles have long straight shafts with a well-marked band a little above the commence- 
ment of the wings. The latter are fairly developed, and merge distally into the 
tapering tip. 
The hooks (PI. XXVIIIa. fig. 13) differ from those of Thelepus in having the 
anterior mucro dorsad of the tip (anterior inferior process). The mucro is also longer. 
A large and a small tooth occur above the great fang, and a series of striae are present 
near the posterior angle. 
The tube, in which the fragment was entirely enveloped, has an internal lining of 
translucent chitinous secretion invested by a remarkable cJievaux-de-frise composed of 
Hexactinellid sponge-spicules, the whole forming a glassy translucent investment. It 
does not follow that the animal specially arranged these for defensive purposes, since 
they were probably the only materials at hand, a supposition the more likely as the 
transjDarent connecting medium had entangled no other organism of note. A certain 
