376 
THE VOYAGE OF H.M.S. CHALLENGER. 
The posterior division has very fine slender bristles with much smaller tips than in 
the first (PI. XXIVa. fig. 5). The whole region resembles that of the European form, 
presenting dorsally the long bristles, which retain the characters represented in the 
figure, and ventrally the soft pads. The bristle-bearing processes have the same clavate 
tips as in SpiochcBto 2 oterus typicus. 
The extreme transparency of the minute hooks on the lateral processes renders their 
detection difficult, and the serrations on the anterior margin are almost invisible. In 
shape these hooks resemble those of Pkyllochcetopteriis socialis. represented by Claparede, 
though the figure appears to be inverted. 
The tubes of this species range from fragments up to 280 or even 380 mm. in length, 
with a diameter of about 2’5 mm. They are for the most part semiopaline or semi- 
translucent, and often ringed for considerable distances, e.g., 2 or 3 inches. Some 
are slightly tinted of a browuiish-yellow. Though somewhat brittle they are very 
elastic, and particles of mud are sent flying considerable distances in tearing the tube, or 
in other manipulations. So very few of the tubes are perfect that the precise condition 
of the anterior aperture is uncertain, but it would seem to be slightly everted. Toward 
the posterior end, again, a very neat diaphragm (PI. XLV. fig. 10a) with a minute aperture 
in the centre occurs ; and above the septum is occasionally a collection of the small ovoid 
faecal masses and other debris, so that this region of the tube is rendered opaque. 
Many of the tubes are forked, an appearance which is due to the fracture of the 
tube, and the continuation of the latter, not by the union of the broken ends, but by 
the secretion of a new^ piece wdth which the old tunnel is continuous; wdiile the broken 
fragment has its channel closed, and it remains adherent apparently as a useless 
process. This seems to occur both anteriorly and posteriorly, as tubes are found wuth a 
diaphragm in each limb of the fork. The bifurcation is thus only apparent, not actual. 
A few of the tubes show a series of such regenerations, so as to resemble the polypary of 
a Sertularian. 
Structurally the tube is composed of the usual chitinous secretion. Immersion for 
some hours in caustic potash causes it to shorten and enlarge, as w^ell as to become more 
opaque and brownish, while similar treatment with hydrochloric acid renders it rather 
more translucent and softer (the tube being flattened), but does not disorganise it, for 
its toughness is little altered. Ordinary chemicals, indeed, have comparatively slight 
efiect on it. 
Few parasitic structures are attached externally to the tubes. In one instance an 
Ascidian with a translucent papillose test w^as fixed to two adjacent tubes ; and amongst 
a group of small yellowish ova were several sand-tubes containing an amphipodous 
Crustacean, after the manner of the British Siphoncecetus typicus, sp. Bate. 
Besides the rightful owner of the tube, a commensalistic form, Polyiioe ocellata, is 
present in considerable numbers. As a ride, the PhyllochcBtopterus could not be obtained 
