TREATMENT OF DISEASES CAUSED BY WORMS                45

days or longer to pass all of the bots destroyed by a drug, as the bots may
wander around in the cæcum for a long time before leaving it.

      Our failure to remove worms may be explained in various ways, and there
are some failures for which we have no explanation. In some cases we have no
known anthelmintic with the selective action necessary for the removal of the
worms. In general, the larger worms in the digestive tract are more readily
removable than the smaller, a thing which Railliet and the writer have both
noted previously, The writer would be inclined to think that the greater size
is actually a reason why larger worms are more readily removed, since a worm with
a diameter relatively large in comparison with the size of the lumen of the tube
in which it lives is more certain to have at least part of the body projecting out
of the mucus and other contents of the tube, and hence accessible to anthelmintic
attack. The difficulty in removing the smaller worms appears to be associated
with the fact that their small diameter permits them to lie in contact with the
mucosa with a layer of mucus and other material above them which mechanically
prevents anthelmintics from coming in contact with them ; or. these small worms
may be actually embedded in the mucosa, or buried in nodules out of reach of
drugs passing through the lumen of the digestive canal. Similarly, larger worms,
such as ascarids, may be inaccessible to anthelmintics if they are in the bile ducts
or pancreatic ducts.

      Partial failures follow in part from the fact that we have no known drugs and
developed methods of procedure which result in 100 per cent. removal of worms.
in 100 per cent. of animals treated, for any species of worm or host animal.
Drugs which remove on an average over large series of cases from 90 to 100 per
cent. of any kind of worms may be regarded as excellent. In default of such drugs
we must use the best we have until we can develop better drugs.