63 ; Canadian Arctic Expedition, 1913-18 
Examination of the Canadian specimen of A. glacialis serves to emphasize 
the close structural agreement of this species with A. marina. The only differ- 
ences of importance between them are in regard to the number of their chaeti- 
ferous segments—19 in marina, 17 in glacialis, and of their gills—13 pairs in 
marina, 11 in glacialis. The gill axes of this specimen are more elongate than 
those of the types and in this respect they approach those of the bushy type 
of A. marina. The two species appear to exhibit differences in the proportions 
of their prostomial lobes and in the shape of their neuropodial crotchets, but 
these are slight and are based on the examination of very few examples of 
glacialis, so that it is not advisable to lay-too much stress upon these. 
It is interesting to note that while the Canadian specimen of A. glacialis 
approaches in the character of its gill the littoral variety of A. marina, it agrees 
in the annulation of the region between the second and third chaetiferous annuli 
with the laminarian variety of A. marina which has pinnate gills. Examination 
of well preserved examples of A. glacialis would show how far the funnels of 
the excretory organs agree with those of A. marina, and young examples would 
be useful for the comparative study of the chaetae, especially of the neuropodia. 
The present position of our knowledge of A. glacialis would seem to be best 
summed up by the statement that this species has been derived from A. marina 
—from which it is apparently a comparatively recent offshoot—by a reduction 
in the number of chaetiferous segments! and of gills. If this should prove to be 
the only difference, the question as to whether A. glacialis is a distinct species 
or only a variety would require consideration. In 1903 I had to consider a 
similar though simpler case in the species A. assimilis, which is represented by 
a forma typica with twenty chaetiferous segments and thirteen pairs of gills, 
the first on the eighth segment, and by a form with nineteen segments and 
thirteen pairs of gills, the first on the seventh segment, but otherwise their 
characters are identical. Although these differ in this constant and striking 
manner, I decided that it would be better to regard the second form as a variety 
—var. affinis—rather than as a distinct species. The difference between A. 
marina and A. glacialis is, however, a matter of two segments! and indicates a 
greater divergence, affording more justification for specific distinction, and I 
suggest that as A. glacialts has been described as a separate species it may be 
retained as such; it is a convenient designation for those specimens of the genus 
with seventeen chaetiferous segments and eleven pairs of small gills, which up 
to the present have been obtained only on the arctic shore of northwest America. 
1 One of the type specimens of A. glacialis possesses an eightcenth neuropodium on both sides but 
not a corresponding notopodium or gill. 
_ |The number of chaetiferous segments in A. marina is rarely other than nineteen, and when there 
is a departure from the normal there is a much greater tendency for the development of an additional 
chaetiferous segment than for the suppression of one. Out of some thousands of specimens which I have 
examined during the past twenty-eight years, only three have been found to have one segment less than 
the normal; in one of these the gill, the notopodium and the neuropodium of the last (eighteenth) segment 
are missing on one side. 
