5 Karu WILHELM GENTHE 
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cylindrical segments and pass laterad between the stomach and body 
wall. Many fibres spring from the lateral body wall of post-discal 
segment 2 and converge to form a slip. The slips passing around post- 
discal segment 1 and converging dorsad are arranged somewhat like 
the meridians in a hemisphere. In the spaces between them the stomach 
has rows of pits (Fig. 16) and evaginations as will be described later. 
These three muscles which have at least for some distance a 
parallel course act as protrusores of the body, esp. of the terminal 
portion with the cirri and produce its rhythmical movements which 
will be referred to later. In contracting the first one pulls the 
head towards the attached knob and depresses it slightly; the second 
one depresses it more. Thus a kind of rotation is started which 
withdraws the cranial portion of the body from the mantle lips. 
It is completed by the action of the third muscle which, at the 
same time, will tend to stretch out the bent in cylindrical portion 
of the last segments and the cirri resulting in a protrusion of the 
cirri through the mantle lips. 
As antagonists to these protrusors we find a set of three 
retractors (r/—3). The fibres of the first one (r/) take their origin 
over a large area on the sides of the body, converge and pass to 
their insertion just in the knob of attachment. 
The second (72) is a slender but strong longitudinal muscle 
which passes along near the dorsal median line. It inserts ventrad 
and caudad from the knob just where the disk is attached to the 
body. As to its origin it matters not whether we trace it all the 
way from the fifth and sixth post-discal segments, or whether we 
place it at the little notch where the first post-discal segment joins 
the disk and consider the other dorsal muscles which pass from 
segment to segment and finally curve around the bulging first post- 
discal segment as separate. 
The third retractor (r3) finally passes directly from the lateral 
and dorsal portions of the post-discal segments over into the disk. 
A contraction of the first of these muscles pulls the region 
marked // in Fig. 8 towards the knob of attachment, thus counter- 
acting the rotation mentioned above. The others withdraw the 
caudal portion of the body from the mantle lips. 
These muscles are evidently supported by fibres which pass 
from the dorsal side of the second post-discal segment ventrad to 
the succeeding segments. Thus all these muscles act as retractors 
of the body and the cirri from the mantle lips. Taking turns in 
