The Compatibility and Incompatibility Concept as Related to 
Trematodes and Molluscs 1 
Thomas C. Cheng 2 
In this paper I propose to bring together 
significant facts already known to have some 
bearing on the mechanisms which govern or 
influence host compatibility or incompatibility 
during parasitism, to add information based on 
my own recent research, and to present some 
speculations. 
From the broad viewpoint, host compatibility 
may be defined as the expression of the physical 
and physiological (including chemical) states 
of the host and parasite which enable the para- 
site to invade and carry out its life processes, 
including perpetuation of the species. Incom- 
patibility refers to those factors which com- 
pletely or partially prevent the establishment 
and normal development of the parasite. Both 
of these are highly complex phenomena influ- 
enced to one degree or another by a series of 
separate but commonly interrelated factors. 
As working models from which we can seek 
evidence to support or reject various working 
hypotheses, we have chosen to examine the re- 
lationship between molluscs and digenetic trem- 
atodes, since this category of association, unlike 
a number of others, is an example of parasitism 
whether the definition of "parasitism" is 
couched in nutritional, pathological, ecological, 
or immunological terms. Molluscs, except in a 
very few unusual instances, serve as intermedi- 
ate hosts for the Digenea, but this fact does 
not render them less appropriate as experimental 
tools. In fact, inasmuch as it is generally ac- 
cepted that molluscs were the original hosts 
1 The research reported in this paper was supported 
by grants from the American Cancer Society and the 
Office of Research Administration, University of 
Hawaii. This paper was originally presented at a 
symposium sponsored by the Division of Invertebrate 
Zoology, American Society of Zoologists, entitled 
"The Parasites of Invertebrates,” held on December 
30, 1966 in Washington, D.C. Manuscript received 
December 13, 1966. 
2 Department of Zoology, University of Hawaii, 
Honolulu, Hawaii 96822. 
during the evolutionary adaptation to para- 
sitism of the progenitors of modern day di- 
genetic trematodes, one might expect to find 
the occurrence of certain adaptive mechanisms 
more firmly entrenched in them than in verte- 
brate definitive hosts. Furthermore, it is easier 
to simulate experimentally the natural environ- 
mental conditions of molluscs than those of 
vertebrates in captivity, and, with relative ease, 
one can test basic premises on a number of 
species from a variety of habitats, ranging from 
marine to fresh water, and from various atti- 
tudes. 
It has long been known that compatibility 
and incompatibility need not be "all or none" 
phenomena, since both interspecific and intra- 
specific (or strain) differences do occur, as is 
indicated by the rate of parasite development, 
infectivity of the cercariae or metacercariae, the 
number of progeny produced by delayed poly- 
embryony, etc. In fact, an understanding of the 
factors governing compatibility and incompati- 
bility in turn most probably will provide an- 
swers for why these manifestations occur. 
Since the initial host-parasite contact, the 
invasion process, the establishment of the para- 
site within the host, and the escape process are 
distinct aspects of a successful parasitic relation- 
ship (see the review by Cheng, 1967), one 
would expect factors correlated with all of these 
phases to contribute to some degree in regu- 
lating compatibility and/or incompatibility, and, 
indeed, available evidence indicates that this is 
so. In the following paragraphs are briefly re- 
viewed those facts which support this concept. 
Space does not permit the citation of all the 
relevant literature ; therefore, only selected 
studies are cited as examples. 
INITIAL HOST-PARASITE CONTACT 
Recently, Timon-David (1965) has again 
raised the question of the importance of host- 
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