382 DITRYPA ARIETINA. 
1912. Ditrypa arietina, Wollebek. Skrift. Selsk. Krist., Bd. n, No. 18, p. 119, pl. xlvi, figs. 4—9, 
pl. hi, fig. 4. : 
1914. - 4 Fauvel. Campag. Scient. Monaco, Fasc. xlvi, p. 846. 
Habitat.—Dredged in 90—96 fathoms off North Unst, Shetland ; in 90 fathoms 25 miles 
west of the Blasquet, 8.W. Ireland ; in 110 fathoms 30 miles W. of Valentia (J. G. J.). 
It frequently occurs in great numbers, N.W. Coast of Ireland (Vidal) ; on the spines of 
the “ piper” in Shetland ; Plymouth (Crawshay). 
Distribution wide—from the North Sea to the Azores and Madeira; Sweden (Lovén, 
Malmgren, etc.) ; off Norway, 300 fathoms, and elsewhere (Sars, Wollebeek, etc.). A variety 
from Bono Bay, dredged by the “ Porcupine ” in 1870, is characterised by its pale brownish 
tube and smaller size. Off the shores of France (De St. Joseph). In the “ Challenger ” it 
was dredged at 823 metres off the Azores, and by the “ Caudan ” in the Gulf of Gascogne, 
180—500 metres ; Madeira, 20—40 fathoms, in sand (Langerhans). 
The collar, though deep (Plate CX XII, fig. 6), is very thin, and its edges laciniated, 
whilst its surface is marked by the linear streaks in the preparations, caused by the adpressed 
branchie. It is fissured in the mid-dorsal line, but is continuous from side to side across 
the ventral surface. No alar membrane is visible dorsally and it appears to be rudimentary. 
The branchize are of considerable length, arranged in two semicircles of about a dozen 
filaments in each, and appear to adhere closely together as if they were bound by a delicate 
web, though this has not been clearly made out, except at the base. The filaments taper 
from base to apex, and end in a short process which scarcely projects further than the adjoining 
pinnee, though it is twice as thick and presents a more distinct central space, whilst its surface 
has cilia (Plate CX XII, fig. 6a). The pinne!' are long, in a double row, and are so arranged 
distally that they form a nearly even series and thus give a character to the tip of the branchie. 
Like the filament, they possess no skeletogenous elements, though in the filament the cuticle 
is thick and tough. The free terminal process of the filaments may have special branchial 
functions when the animal withdraws into its tube, for they project all round in the space 
below the opercular plug. 
The pedicle of the operculum is long, and springs from the dorsal edge of the left branchial 
fan, and it remains nearly cylindrical to the tips of the branchiz, where it dilates into the 
long and shapely vase with the yellowish-green or dull yellow calcareous plate, 1:20 mm. 
in diameter, at its tip. The projection of the operculum proper beyond the terminal processes 
of the branchiz is noteworthy. In the ordinary spirit-preparations the pedicle lies in the 
midst of the branchial filaments, and is thus in contrast with the conditions usually seen in 
other forms. The distal opercular plate is flat and brittle, presenting under the microscope 
a deep yellow hue, and a minutely cellulo-granular aspect, the margin, however, being 
hyaline. The pedicle is flattened inferiorly, but toward the base of the opercular vase it is 
rounded. 
The body (Plate CXXII, fig. 5), which is 13—16 mm. or more in length in spirit- 
preparations, has not been observed in the free condition, all being moulded by the tube into a 
cylindrical form anteriorly, and only a little tapered toward the tail, which has a short conical 
outline. ‘The anterior region is distinguished by the six pairs of bristles laterally, the achetous 
segment in front, and by the fillet with a median notch ventrally—the representative of the 
' De St. Joseph noted 33—85 pairs of pinne. 
