Vou. 2] Robertson.—Non-Incrusting Bryozoa. 283 
ocecia are much alike both as to size and position, and resemble 
those of Bicellaria. The stalk of these three species shows varia- 
tion tending toward greater specialization. Attention has already 
been ealled to the fact that, as Hincks (’83) observed, the seg- 
ments of the stalk are probably aborted zocecia. Their zocecial 
nature is most plainly shown in NS. ciliata, in which as shown in 
Pl. XII, fig. 68, the distal segment (dis. seg.) of the stalk is not 
unlike a normal zocecium in shape, and in the possession of a 
definite aperture round whose margin seven spines may be 
eounted. This zocecium-like segment, however, does not contain 
a polypide, but instead, a strand of tissue similar to that found 
in other segments of the same stalk. In the remaining species, 
S. occidentalis and S. californica, the zocecial character of the 
segments of the stalk is much less apparent, the aperture on 
each being inconspicuous and the articulation of the stalk being 
much more definite, showing a higher grade of specialization 
than is found in S. ciliata. With the specialization of the seg- 
ments of the stalk there is correlated greater size, both in length 
and thickness, not only of the stalk as a whole but of the indi- 
vidual segments as well. 
Attention has already been ealled to the difference in size 
between the lower and upper segments of the stalk of S. occi- 
dentaus (El wid, fie. 73> Pl. XPV, fic. 76). A similar differ: 
ence exists in the segments of the stalk of S. californica, fig. 78 
representing in outline one of the longer, fig. 79, one of the 
shorter segments, all the figures being drawn to the same magni- 
fication. Furthermore, a distinction is made between NS. califor- 
nica and any other members of the genus in the possession of a 
foot disk by which the colony adheres to the substratum. Fig. 77 
represents the mode of attachment of a small colony consisting 
of a single segmented stalk. Where the stalk approaches the 
substratum it spreads out in a thin membranous disk, strength- 
ened at intervals by chitinous ribs (7b.) which form in the ecto- 
eyst of the lower portion of the stalk and extend to the cireum- 
ference of the disk. A colony may consist of a single stalk 
arising from such a disk and surmounted by a zocecial tuft; or 
as in Pl. XIII, fig. 75, the stalk which forms the foot disk may 
