s when it comes to the surface and then great suffering may
result. The African eye-worm is another example of a parasite causing
mechanical injury only at certain times. It works in the tissues of the
body sometimes for a long while, doing no harm unless it finds its way
to the connective tissue of the eyeball.

It is known that many of the germs which cause diseases cannot get into
the body unless the protecting membranes have first been injured in some
way. Thus the germs that cause plague and lockjaw find their way into
the system principally through abrasions of the skin. Many physicians
have come to believe that the typhoid fever germ cannot get into the
body from the intestine where it is taken with our food or drink unless
the walls of the intestine have been injured in some way. It is well
known that of the many parasites that inhabit the alimentary canal some
rasp the surface and others bore through into the body cavity. This in
itself may not be a serious thing, but if the mechanical injury thus
caused opens the way for malignant germs, baneful results may follow.
Even that popular disease appendicitis is believed to be due sometimes
to the injury caused by the work of parasites in the appendix.

Parasites may cause morphological or structural changes in the tissues
of their hosts. The stimulation caused by their presence may result in
swellings or excresences or other abnormal growths. Interesting examples
of this are to be found in the way in which pearls are formed in various
mollusks. In the pearl oysters of Ceylon occur some of the best pearls.
If these are carefully sectioned there may usually be found at the
center the remains of certain cestode larvae whose presence in the oyster
caused it to deposit the nacreous layers that make up the pearl. Other
parasites cause similar growths in various shellfish. The great
enlargements of the arms or legs or other parts of the body seen in
patients affected with elephantiasis is an abnormal growth due to the
presence of the parasitic filarae in some of the lymph-glands where they
have come to rest.

Finally, the parasite may exert a direct physiological effect on the
host. This is evident when the parasite demands and takes a portion of
the nourishment that would otherwise go to the building up of the host.
Sometimes this is of little importance, but at other times it may be a
matter of life or death to the infected animal. The physiological
effect produced ma