DIBOTHRIA FROM YELLOWSTONE NATIONAL PARK. 7l 
era! or the central aquiferous vessels on the one hand, and the marginal canals on 
the other, ' 
Calcareous bodies . — These are numerous, especially in the anterior regions of the 
body. They are confined for the most part to the central core and to the peripheral 
region. In the latter they are nearly all found between the cuticle and the peripheral 
vascular layer. In the median regions of the body they have almost disappeared from 
all parts except the peripheral layer and there they are not abundant. In sections 
from the posterior parts of the body, where the reproductive organs have begun to 
develop they are yet more sparsely scattered and are confined almost exclusivelj^ 
to the peripheral region. In size these calcareous particles vary. The larger ones 
measure about .008 by .01 millimeters in the two diameters. Most of them are cir- 
cular or oval in outline, but some are irregular, A great many of them show a con- 
centric structure in optical section. 
Reproductive organs . — In these ligulm from the abdominal cavity of the sucker the 
reproductive organs have reached a comparatively advanced stage of development. 
Both longitudinal and transverse sections from the middle and posterior parts of 
the body show clusters of nuclei which are deeply stained by carmine. These clusters 
lie in the inner layer of longitudinal muscle fibers, near one of the lateral faces. In 
longitudinal sections made parallel to the lateral faces of the body they are seen to be 
arranged along the median line in a somewhat zigzag row, and correspond in posi- 
tion to the genital openings which are also revealed in those sections which are 
carried through the superficial layers of one of the lateral faces. In transverse sec- 
tions each of these clusters of nuclei is seen to consist of two clusters lying side by side 
(plate XXV, fig. 1), They are the rudimentary genital organs. The one will, in the 
adult, give rise to the cirrus and its pouch, and the spermatic reservoir; the other to 
the vagina, uterus, ovary, etc. In the peripheral region the vitellaria are plainly" 
indicated by a nuclear layer, which is separated from the cuticle by a thin layer of 
longitudinal fibers and extends to the inner layer of longitudinal muscles. The nuclei 
which lie in the interstices of the external layer of longitudinal muscles and those 
which are so abundant in the vascular layer in the anterior part of the body evidently 
contribute to the formation of the vitellaria. 
The beginning testes are plainly indicated in transverse sections of the postero- 
median and posterior regions of the body. These consist, in each section, of a series 
of a dozen or more nests of nuclei lying towards each margin, extending from near the 
margin towards the center a distance equal to about one-third the diameter from 
margin to margin (plate XXIY, fig. 3d). These lie in the central core. One of these 
nests of nuclei is shown in section, highly magnified in fig. 42, ifiate XXIV, 
Nuclei, which in the anterior regions of the body are more or less abundantly dis- 
seminated among the muscular and fibrous layers, are, in the median and posterior 
regions, confined to the peripheral and central regions, where theyhave alreadybegun to 
collect to form the genitalia. The transverse fibers which in the anterior regions are 
abundant and strong, especially in the central core, and circular layer, are in large 
measui'e reduced to very fine fibers in the median and posterior regions. The various 
tissues of the body have already in great degree been absorbed to contribute to tbe 
formation of the genitalia. The inner layer of longitudinal muscles appears to have 
suffered the least from absorption thus far. 
