F. J. Meggitt 
381 
which run internally and fuse with one another in the centre between 
the three arms. 
As the vessels proceed anteriorly, this definite arrangement becomes 
lost, until just posterior to the neck, the ventral vessels are represented 
only by a plexus of minute branches. As the vessels pass into the neck, 
the excretory plexus representing the ventral vessel merges into two 
large transverse vessels uniting with one another just external to the 
dorsal vessel. Text-figure 1 presents the typical appearance here of the 
excretory system in a tri-radiate Cestode. From this point, the dorsal 
vessel (a) runs practically in a straight line into the scolex. The course 
of the ventral (b) is more complicated (PI. XVIII, fig. 4). Starting lateral 
and slightly external to the dorsal vessel, it runs half-way to the scolex, 
there to bend upon itself and retrace its course to a point still more 
lateral to the dorsal vessel and on the same side of it as before. Again 
it bends round upon itself, and this time goes forwards in a straight 
line to the scolex where it breaks up into an excretory plexus between 
the sucker and the outer margin of the proglottis. The branches of this 
plexus reunite into a single vessel (6'), which runs posteriorly, increasing 
in size, to a point, the same distance from the dorsal vessel as the 
furthermost point it reached in the first part of its course, only this 
time on the opposite side. From this point it again runs forward 
into the scolex, breaking up into an excretory plexus as before, but 
going to the sucker on its own side of the dorsal vessel. The posterior 
margins of the six suckers are thus supplied by the ventral vessels, the 
suckers on either side of the dorsal vessel forming a pair supplied by the 
corresponding ventral vessel. As before, the branches composing the 
plexus reunite into a large one ( b") which runs posteriorly half-way 
through the neck and then proceeds anteriorly again, external to and 
closely following the course of the dorsal vessel, into the scolex. With 
the exception of the portion before the first bend, the whole of its 
course lies external to the main longitudinal nerve trunk. The appear¬ 
ance in this part of the scolex is shown in Text-figure 2. 
The excretory system in the scolex may be divided into three parts, 
each occupying a third of its length. Sections through the junction of 
two of these parts show a regular and symmetrical arrangement, 
sections between the junctions show only a network of vessels. In 
order accurately to describe these systems it is necessary to refer to each 
individual vessel as the majority of them have different courses. 
Each of the three ventral vessels breaks up into an excretory plexus 
once more. The dorsal vessels on the other hand continue without a 
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