MAMMALIAN CESTODES. 15 
§ Comparison of the Young Plerocercoids with the Buds. 
As has been already observed, there are no free living stages of 
this worm in my possession which are younger than the specimens 
described in the foregoing section. But it hasalso been suggested 
as a possibility that the buds formerly described by me may be a 
stage antecedent to those which I have termed the young plero- 
cercoid. When freed from the fully developed asexual worm they 
may drop off into the liver ducts and there increase in size—prin- 
cipally in breadth, for the length of the longest buds is not far 
removed from that of the smallest free plerocercoid—and, after a 
' certain alteration in structure, show the characters of the maggot- 
‘like larvee which have been dealt with in detail in the last few 
pages. I do not think, however, that this can be the case, failing 
at any rate further evidence. For the most “mature” of the 
buds which I studied in some detail for my earlier communication 
upon this genus has a much more “adult” structure than has 
the youngest of the plerocercoids described in the present paper. 
And if we are to assume that the young plerocercoids described 
here develop gradually into the large sexless worm, as I think is 
fairly certain, this possibility is rendered still less likely. 
I shall now indicate the differences in structure between the 
_ plerocercoids and the buds which oppose themselves to this com- 
parison. The various stages in the development of the buds shown 
in text-fig. 116 * of my earlier paper seem to show that the bladder 
is the first part of the worm to appear and that the vermiform 
_region is an outgrowth of this. For the buds are at first bladder- 
like outgrowths of the stock, and then develop a process at the free 
end which gets to be more important. Now in the plerocercoids 
the whole worm is at first also little more than the bladder region ; 
but this is converted into the more mature individual by a pro- 
portionate suppression of the cavities which together constitute 
the bladder-like region, and the gradual conversion of the greater 
‘part of the worm into a more markedly vermiform body. Traces 
of the bladder-cavities, however, persist in the latter up to the 
anterior end. ' This difference of mode of growth does not, how- 
ever, exhaust the differences observable between the two series 
of young worms. The bladder-cavity itself differs in thetwo. As 
already described, it has no proper walls in the free young plero- 
cercoids, while in the buds it has not precisely a definite wall, 
but the inner layer is thickened and it is sharply marked off from 
the empty space which it encloses: it is, in fact, like some of the 
cavities lettered C in the young free worms. 
There is, however, the tube which I have lettered « in my 
figure of a ‘transverse section through a bud}, which is in 
structure very like the spaces lettered A in the transverse section 
through the young plerocercoid (text-figs. 2, 3). The former is, 
* P.Z.S. 1912, p. 824. 
+ Ibid. text-fig. 116, p. 833. 
