1911-12.] Structure and Affinities of Branchiomaldane vincenti. 63 
the dorsal epithelium, such as is present in late post-larval and in adult 
examples of Arenicola * 
The next segment is almost without chrntse, and is evidently homologous 
with the achmtous body segment of Arenicola, for, in both Branchio- 
maldane and Arenicola the oesophageal connectives unite near the middle 
of this segment. It is followed by the chsetigerous segments, the 
number of which depends on the stage of growth attained; the largest 
number observed is fifty-one (see fig. 1). The terminal pygidium is 
usually bluntly conical, but its shape varies in different specimens accord- 
ing to the condition of extension or retraction at the time of fixation. It 
is slightly notched terminally, its end being four- or five-lobed. 
Each chsetigerous segment bears notopodial chsetse and neuropodial 
•crotchets. The notopodia and the short f neuropodia are comparatively 
feebly developed, being little raised above the general surface. In all the 
preserved specimens examined the notopodia are not well-marked conical 
outgrowths, such as are present in Arenicola, but are only slight rounded 
elevations on which the aperture of the notopodial chsetal sac, and the 
tips of the capillary chsetse, are seen. These chsetm resemble, in their 
general structure, those of a post-larval Arenicola ecaudata, but in 
each notopodium about one-half the chaetse have the distal fourth bent 
on the proximal part at an angle of about 150°. The crotchets, which 
are always few in number — from two or three in the first two and last 
segments, up to nine or ten in most of the branchial segments, — are of the 
same general form as those of Arenicola, but the rostrum is very sharply 
pointed. As in Arenicola, a crotchet of different form — with more elongate 
shaft, smaller rostral region, and more distally situated nodulus — is present, 
but for a short time only, in a few of the last-formed notopodia. 
Most of the chsetigerous segments are annulated. But the annulation of 
the anterior and middle segments is much less definite, and the number of 
annuli per segment is less constant than in Arenicola. In the larger 
specimens the annulation becomes obvious usually about the third segment, 
which is subdivided into four or five rings ; each of the succeeding segments, 
as far back as about the twentieth, shows from five to seven rings. In the 
* In a young post-larval A. ecaudata, 5’5 mm. long, with 46 chsetigerous segments, there 
is no nuchal invagination, hut only a faint groove ; in an older example, 8 - 5 mm. long, 
with 56 chsetigerous segments, there is between the prostomium and peristomium a distinct 
invagination of the epithelium, forming the incipient nuchal organ. Examination of 
sagittal serial sections of a mature specimen of B. vincenti , 10 mm. long, shows that a nuchal 
invagination is not present. 
+ Young examples of A. ecaudata , of about the same size as the specimens of B. vincenti 
described above, have considerably longer neuropodia. 
