DIBOTHEIA FROM YELLOWSTONE NATIONAL PARK. 
69 
sectious, suffer most cliaiige, being reduced to slender filaments. Their substance in 
the inner core is plainly lost to the genitalia which are already taking shape. 
Vascular system . — The vascular system is represented by two sets of vessels which 
may, for convenience, be designated (1) peripheral, and (2) central or centro-marginal. 
The i)eripheral vessels constitute a net- work of vessels which lies in what was called 
above the porous or vascular layer, between the subcuticular granular layer and the 
inner longitudinal layer. In this set of vessels there are a great many longitudinal 
vessels with numerous anastomosing branches. In both longitudinal and transverse 
sections many branches were seen to leave the iieripheral layer, and penetrating the 
longitudinal muscle layer debouched into one of the longitudinal central vessels of 
the inner core. Hear the anterior end of the body the peripheral vessels are most 
numerous, forming there an intricate net- work. Towards the middle of the body they 
become reduced in number, and assume more the character of longitudinal vessels. 
This character is retained near the posterior end, where the number of these vessels 
shown in transverse sections along each lateral border of a section is five or six. In 
some of the first sections of the anterior end the peripheral vessels were seen si)read over 
the central part of the sections, showing that the vascular layer closes in the anterior 
end of the body along with the cuticular and subcuticular layers (plate XXIV, fig. 1). 
Their disposition at the extreme posterior end was not made out. 
The second set of vessels are the usual centro-marginal vessels peculiar to the 
cestods. Of these there are two sorts, corresponding to what may be seen in adult 
Dibothria. In brief, these two sorts may be characterized as follows : The first sort 
consists of two conspicuous vessels, one lying towards each margin, without proper 
walls, excepting a few circular fibers and numerous nuclei, the lumen being filled with 
spongy tissue, which does not stain strongly with carmine, but contains a few nuclei, 
similar to those which surround the vessels (plate XXIY, fig. 2, a d). The second sort 
consists of a varying number of vessels more numerous towards the anterior end, 
with iiroper walls and an open lumen (plate XXIY, fig. 2, h). 
I shall, for convenience, speak of the former as the marginal canals, and the latter 
as the aquiferous vessels. 
The marginal canals make their appearance in sections, made very near the an- 
terior end, as two nearly circular spots of spongy or areolar unstained or slightly 
stained tissue near the center of the sections. A line joining their centers corresponds 
with a line drawn from margin to margin. They lie directly between the lateral 
notches which mark the rudimentary bothria. At first they almost coincide, so that 
they appear to communicate. They soon separate, however, as they are traced poste- 
riorly, and at a distance of 2 or 3 millimeters from the anterior end they are situated 
at about the same distance from the margins as they are from each other. Near the 
middle of the body they are relatively nearer the margins, the distance from one canal 
to the other being about one and two-thirds times the distance from acanal to the nearest 
margin. Towards the posterior end they approach relatively nearer the margins. 
These canals preserve their distinctive character throughout their whole extent. They 
do not in any way resemble the aquiferous vessels. Instead of being hollow tubes, as 
is the case with the latter, they are filled with a spongy connective tissue, which is 
very slightly affected by carmine staining. They are nearly circular in outline, and 
are limited by a few circular fibers in which there are numerous nuclei, in some of 
