350 HYDROIDES NORVEGICA. 
when coiled. No special chordoid or other axial structure 1s present, but the cuticle is very 
thick. The pine are rather short, devoid of a special stiffening apparatus, and extend to 
the tip, partially overlapping the terminal process. They taper a little from base to apex. 
In life the branchize are spotted with crimson and have a pinkish hue from the colour of the 
pine. 
The pedicle of the operculum has a transparent but tough investment of a cellular mem- 
brane (mainly hexagonal). Internally is a yellowish fibro-granular and apparently muscular 
cylinder in the preparations. The distal funnel or wheel of the operculum is brownish, 
horny, and when viewed from above has somewhat the aspect of a Chinese umbrella, with 
a fissured and spinous rim of sixteen to twenty or more processes. Moreover, the upper 
surface of the wheel is armed with spikes (Plate CX-XI, figs. 6 and 6b), which form a series of 
rings round the central area. In lateral view it is vase-shaped, with a graceful inclination 
outward and upward from the narrow stem. Hach of the rays forms a stout, tapering process, 
terminated by a thin flexible tip, each side being armed with three or four stiffer and shorter 
spines, a double contour being visible for fully the basal half. The longest lateral spines 
are, as a rule, the distal pair, since they have a wider area to guard. This elegant horny 
apparatus takes the place of the calcareous operculum of other forms, and the flexible tips 
of the rays apparently yield to a certain extent in withdrawal into the tube, causing 
the spies to approach each other more closely, and thus efficiently protecting the aperture. 
In some large examples the dorsal and lateral spikes are bifid; various smaller spikes occur 
in the intervals, and an imperfect ray is intercalated between two of the others. 
The second tier of the operculum (Plate CX-XI, figs. 66 and 6c) forms a gracefully fluted 
cup, with about twenty-seven crenations, each separated from its neighbour by an involution 
of the tough integument enclosmg hypodermic cells and granules. The muscular fibres in the 
centre of the stem expand as they approach the lower operculum, on the centre of which 
they pull, and so fix the elastic crenate margin on the sides of the tube. The protection of 
the aperture is thus doubly secured, for the distal spimous shield closes over the crenate cup 
and exposes only the tough spiny rays to an ageressor. It requires great force to detach 
this cup from the opercular stalk, apparently from the toughness of the cuticle. The rudi- 
mentary organ of the other dorsal edge is often minute, resembling a Loxosoma, and springs 
from the fan near the base of the first filament of the sides, frequently the left dorsal 
filament of the branchial fan (Plate CX XXIII, fig. 1). It has the form of a stout pedicle, 
tapering a little distally, and having the small truncated operculum at the tip in the form 
of an inverted cone, without appendage. 
The dorsal edge of the collar (Plate CX XI, fig. 6a), which is supported by the first bristle- 
bundles, forms the commencement of the alar membrane, which passes backward over the 
anterior bristles to their termination, beyond which its thin edge projects ventrally as a 
flap of a bluntly conical outline. In the preparations, as a rule, the anterior edges of the 
alar membrane approach each other, whilst the posterior margins, as they trend to the ventral 
surface, are wide apart. ‘The bristle-tufts stand clear of the membrane on each side, and the 
free ventral flap is ample. The dorsal surface in this region is somewhat flattened, whilst 
the ventral is slightly convex and marked by a median groove. In the second region the 
dorsum of the body is slightly convex throughout, and the ventral surface is but little 
flattened, though marked by a median groove which runs to the tapered tail, with the 
