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Tetrarhynch us er in ace us 
apparently from the dorsal to the ventral surfaces of the proglottis. 
About midway on the course of each is the muscle cell, which has the 
form of a plate along one side of which the fibre is embedded. The cell 
has no membrane and is reticular in structure. The nucleus is large; 
has a prominent nucleolus; and a chromatic network restricted to the 
peripheral parts. 
The excretory organs. I describe here the main canals only and not 
the finer terminations, nor the terminal flame cells. There are four 
main canals in each proglottis, two of which run longitudinally along 
each lateral margin. In transverse section one of these canals is always 
larger than the other, this one is the more ventral of the two. The 
canals are situated between the testicular follicles and the vitelline 
glands, but usually the presence of the canal, with the large bundles of 
longitudinal muscle fibres which are situated adjacent to them, causes 
the vitelline glands to be less abundant at the lateral margins of the 
segment. The canals are not straight but are usually loosely coiled—an 
appearance however which may be due to a longitudinal shortening of 
the proglottis after fixation, for when the Cestode is killed in fresh 
water and fixed in weak formalin the canals may be quite straight. 
This condition may however be produced by the absorption of water by the 
tissues of the worm, and perhaps the accumulation of this in the excretory 
system. As a rule the canals are compressed in transverse section and 
have walls which are rather thick, and of no recognisable structure; but 
sometimes the outline of the canal in transverse section may be perfectly 
circular and the wall itself may be exceedingly thin. These lateral 
canals, certainly the larger one of each side, run without interruption 
from one segment to the next, and at the junction of two adjacent 
proglottides there is an anastomosing transverse branch which connects 
the canals of the two sides. When the segments separate from each 
other the cleavage takes place along the middle of this transverse 
anastomosis, so that the canals then open to the exterior at each side of 
the segment. There are no terminal vesicles, nor valves at the origin 
of the transverse anastomoses. 
VII. The 'mature proglottis. 
It is quite unnecessary to describe in detail the anatomy of the ripe 
proglottis since the relations of the various organs are very much 
the same as in the immature segment. The differences consist of the 
progressive development of the uterine sac as the segment enlarges; and 
