177 
The arrangement of the protomesenteries in Morten se n’s 
specimen is very anormal. Only 5 or probably 4 protomesenteries 
are present, the one directive mesentery is certainly absent. This 
abnormity is also on the right side accompanied by an irregular 
development of the proto¬ 
mesenteries 2 and 3 (p 2 
and ps). As it is easy to 
distinguish the metamesen- 
teries M and m from each 
other and these mesenteries 
from the B- and b-mesen- 
teries, owing to the diffe- 
rent structure of their fila¬ 
ments, we are able to de- 
termine the nature of the 
protomesenteries 2 and 3 
by comparing these mesen¬ 
teries with the metamesen- 
teries. Starting from the 
directive mesentery (dm) 
we find (fig. 6) on the right 
side a short sterile mesen¬ 
tery with the structure of 
a metamesentery B, then 
a fertile mesentery consi- 
derably longer than the fol- 
lowing metamesentery (Mx) 
but in its structure agree- 
ing with this mesentery. On 
the left side of the direc¬ 
tive mesentery there is a 
' rather long fertile mesen¬ 
tery of the metamesenterial 
Af 
Fig. 6. Cerianthus (?) mortensenii. Diagram 
of the arrangement of mesenteries. 
type m, then a sterile mesentery agreeing with the metamesenteries 
B and b, Applying to the protomesenteries 2 and 3 the terms of 
the metamesenteries and supposing that only one protomesentery 
:(one directive mesentery) is suppressed, the formula of the proto- 
:mesenteripc thnc io r ..v, a: __ ^ _ 
luiniuid Ul IIIC proio- 
jmesenteries thus is B (b), m, directive mesentery, B, M. On the one 
1 tt: j 1 - 
^idensk. Medcl. fra Dansk naturh. Foren. Bd. 75. 
12 
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