SPIRORBIS BOREALIS. 413 
The operculum (Plate CXXII, fig. 10 and Plate CXXXII, fig. 4d), derived from the 
first left branchia, is saucer-shaped, with a massive peduncle, is calcareous, and often infested 
by Diatoms and other structures. De St. Joseph found Cothurnia maritima growing on the 
operculum, and reproducing by fissiparity. 
The body is dark brown in front from the alimentary canal, reddish orange posteriorly 
and 3 to 4 mm. in length. The pigment is arranged in irregular granular masses. The 
anterior region has three bristled segments, the first or collar-series having characteristic 
tips. The first part of the posterior region of the body is not segmented, though ciliated, 
the total number of segments in the region being eighteen to thirty-two, and terminated by 
the anus with a rounded papilla on each side. Hach seement has two geniculate bristles. 
The segmentation of the posterior region of this species is better marked than in S. granu- 
latus. In the first segment are two granular excretory organs from which ciliated canals 
jo to form a single duct opening at the base of the branchie. 
De St. Joseph describes the alimentary apparatus as composed of an cesophagus in the 
anterior region, a large brown cylindrical stomach immediately behind, followed by a sinuous 
intestine. He found spicules of sponges, diatoms, amongst others Sphenosira catena, im 
the stomach, all probably carried in by muddy currents. A blood-sinus encircles the canal 
with lateral branches and ceeca in each segment. In some a pale green hue occurs below 
and at the sides of the stomach, and continues more faintly along the intestine to the vent. 
This apparently is due to the greenish blood of the sinus, since it disappears when a hernia 
of the stomach ensues. Lateral branches (segmental) leave the vessel at the out. Gregarines 
similar to those of S. granulatus occur im the canal of this species, and they show the 
same active contractions and flexures. 
The first or collar-bristles (Plate CX XXII, fig. 4) arise from a nearly cylindrical setigerous 
process, and difier from those of Spirorbes granulatus, L. Their shafts are less robust, though 
straight, and they also dilate distally into a flattened process at the commencement of the 
tip, but no fine serrations occur on the convex edge, the whole being occupied by two to five 
(Levinsen shows seven) rather coarse serrations, which slightly increase in size in lateral 
view from below upward, the strize on the flattened end of the shaft making them conspicuous. 
Then the smooth edge of the ~ bite ” occurs as it rises to the somewhat bellied blade beyond, 
which, however, soon tapers to a delicate extremity, usually with a marked curvature. The 
bellied condition of the blade beyond the “ bite ” 1s a diagnostic feature in comparison with 
the bristles of Sp. granulatus, as also is the comparatively coarse edge, the serrations on 
which point upward and outward, so that a certain uniformity prevails. The basal part, 
moreover, is separated only by a sheht step from the distal, and the serrations on it sometimes 
show a different angle. Such, however, diverges from the figure of the bristle given by 
De St. Joseph, in which the distal serrations are at right angles to the axis of the tip, a 
condition probably due to an indifferent microscope. The step of this bristle shows a distinct 
character when compared with the gap generally present in the collar-bristles. The shorter 
bristles are simple, tapering forms. The web at the base of the first series of bristles is 
seen to be double, and thus becomes homologous with the dilated spinous region at the 
distal end of the shaft in other forms. About the same number of simple tapering bristles 
occur with them in the tuft. 
The bristles of the second setigerous process are simple straight forms, the tapered tips 
