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381 
in the pressure of this fluid. The lumen of the sheath and that of the 
bulb are continuous so that when the latter is contracted the fluid in the 
sheaths presses upon the part of the wall just where it is invaginated 
inwards. The mechanism involved is obviously similar to that which 
leads to the evagination of the proboscis in Nemertine and Acantho- 
cephalan worms. 
There are four series of muscles in relation to the proboscides;— 
(1) Extrinsic tangential fibres. These are represented in PI. XIX, 
fig. 4. They are found in the region of the non-evaginable proboscis 
sheaths. They are fibres which originate in the walls of the bothridia 
and the posterior part of the scolex and which are inserted tangentially 
into the walls of the sheaths throughout the greater part of the lengths 
of the latter. Their function is obviously that of holding in position the 
non-evaginable parts of the proboscis sheaths. 
Fig. 4. Two diagrammatic transverse sections through the neck of the worm. The 
upper section passes immediately behind the proboscis bulbs; the lower is just before 
the region of segmentation. 
(2) Extrinsic longitudinal fibres. These are represented in PI. XIX, 
figs. 3 and 5 and in Text-fig. 4. They are relatively coarse fibres which 
are inserted in the outer parts of the walls of the bulbs along the 
greater parts of the length of the latter. Towards the posterior part of 
the bulb these fibres are gathered up to form four roughly crescentic 
bundles the convex surfaces of the crescents being external (Text-fig. 4). 
A little way further back in the neck of the Cestode these bundles 
flatten out and the two dorsal ones fuse together to form a sheet of 
