The Rot in Sheep. 
97 
external and easily recog^nisablo characters for the generic sub- 
divisions of the Trcmatodc order accordino; to the numbers and 
positions of the suctorious orifices and cavities. " When there is 
only a single one, it constitutes the ^cnns Monostoma ; when there 
are two, which are terminal or at opposite ends of the body, you 
have the character of the genus Amphistoma ;* when the posterior 
of the two suckers is not terminal, but on the inferior surface of 
the body, it constitutes the genus Distoma ; three suctorious 
cavities characterise the genus Tristoma ; five the genus Penta- 
stoma ; and a greater number that called Poli/storna." 
Form and Size. — The Distoma hepaticum varies in size in the 
same animal, according to the age of the entozoon. Although 
this is the case, it is a singular circumstance, hereafter to be 
explained, that no distomata are found, even in long-existing 
cases of rot, so small as to warrant the belief that they had been 
hatched within the biliary ducts. The form of the entozoon is 
that of an oblong oval, flattened from side to side. Its greatest 
breadth is anteriorly, immediately behind the central sucker, 
from which point it gradually tapers to its caudal extremity. 
When fully developed, the distoma will attain a length of an 
inch and quarter, and a breadth of half an inch at its widest 
part. Many of the smaller specimens, however, do not bear the 
same proportion between their length and breadth, being some- 
what rounder in form. It is, however, to be borne in mind, that 
on being removed alive from the biliary ducts, the creatures are 
seen to contract themselves, so as to appear very much smaller 
than they really are — a circumstance which has often led to an 
incorrect conclusion with regard to their real size, and conse- 
quently as to their age, and the length of time they had been 
located within the ducts. 
Fig. 2. 
Fully-developed Distomata. 
We here insert an engraving (^Jig. 2) of two distomata of ma- 
tured growth, which will assist our exposition. One of them is 
' ■ . — - — ■ 
* The fluke thus named is frequently met with in oxen and sheep, attached to 
the mucous surface of the rumen, in -which situation it appears to be unproductive 
of mischief. — AnTHOB. 
VOL. XXIII. 
H 
