ANATOMY OF TANIA MEDIOCANELLATA. 13 
tremely irregular in shape, more laterally they are somewhat 
pear-shaped, but flattened where they abut upon this boun- 
dary, while those whose discharging tubes are horizontal are 
narrow elongated and lobulated (fig. 2) ; in fact their outline 
from an original sphere or oval is ultimately determined by 
compression from within or from contiguous external struc- 
tures. Their contents are closely aggregated vitelline masses 
or germ-cells (fig. 13, e), these necessarily varying in size 
and aspect with the degree of maturity of the zooid will be 
hereafter detailed. 
As compared with the seminal glands the ovarian glands 
are differentiated from them by the nature of the encapsulating 
membrane, largeness of size, largeness and divergence in 
structure of the component elements. In both systems, how- 
ever, the analogy is complete of the special element-producing 
masses being contained within cecal dilatations of the con- 
veying tubes. 
Water-Vascular System.—Traversing the segment on 
each side, parallel to the lateral edges at the distance of ;'; in. 
within the visceral boundary, is a canal average ='5 in. in 
diameter (fig. 1, ¢). The lumen of the canal throughout the 
segment is far from equal and the course is marked by slight 
undulations. At the lower end of each segment each longi- 
tudinal canal swells out into a pear-shaped dilatation =, in. 
by => globular end towards the side of the segment and 
narrow end continuous with the transverse water-vascular 
canal (fig. 2, d,e). This transverse or connecting branch, 
traversing the centre of the visceral space at the lower boun- 
dary of the segment, is oval in outline, straight in its course, 
but tapers slightly towards the centre; its lumen measures 
z's In. by zy. Immediately below it is the fibrous diaphragm 
separating one segment from the other. ‘The wall of the 
canals is thin, composed of delicate longitudinal and circularly 
arranged fibrous tissue (fig. 13, a), and there is no appear- 
ance of any intercepting medium to the most free passage 
of fluids in any direction. ‘lhe longitudinal canals are not 
uncommonly pushed on one side by the passing genital ducts, 
while the inner wall is flattened and often globularly pro- 
jected inwards by the distended ovarian glands which abut 
on it. Those of one segment are continuous with those of 
the preceding and succeeding ones, varying only from them 
in gradation of diameter ; the transverse branch is special to 
each zooid. ‘The globular end of the dilatation at the junc- 
tion of the canals comes into close contact with the inflected 
body structures at the union of the segments, but apparently 
there is no communication between it and the external sur- 
