Compatibility in Trematodes and Molluscs — Cheng 
155 
draw unwarranted conclusions relative to the 
lethality of parasites, particularly helminths, 
based on histopathological studies. 
ESCAPE PROCESS 
Our present knowledge concerning the pas- 
sive and/or active mechanisms employed by 
cercariae while escaping from their molluscan 
hosts is based primarily on studies carried out 
on the human-infecting species of schistosomes 
(see reviews by Probert and Erasmus, 1965 ; 
Cheng, 1967), although some information is 
available on Fasciola hepatica (Kendall and 
McCullough, 1951), Neodiplostomum inter- 
medium (Pearson, 1961), and Cercaria X 
(Probert and Erasmus, 1965). All these studies 
have been concerned with the processes which 
make possible successful escape, and hence, in a 
manner of speaking, govern compatibility. On 
the other hand, if some factor or factors within 
the mollusc interfere with cercarial escape and 
thus prevent the parasites from continuing their 
normal course of development, this factor or 
factors may contribute to host-incompatibility. 
In seeking evidence for this hypothetical pos- 
sibility, one should be cautious in distinguish- 
ing between consistent barriers, either structural 
or physiological, which prevent escape and in 
so doing endanger the perpetuation of the 
parasite species, and an occasional accidental 
arrest of the escape of a few cercariae. Examples 
of the latter include the report by Cheng and 
Cooperman (1964) that occasionally an escap- 
ing cercaria of Glypthelmins pennsylvaniensis 
may accidentally migrate into the foot mus- 
culature of its snail host, Helisoma trivolvis , 
and become encapsulated, and the report by 
Pan (1965) that some escaping S. mansoni 
cercariae do become trapped in the loose vas- 
cular connective tissue of A. glabratus and die. 
Yet, in the latter case the frequency of this 
event suggests that it is a normal occurrence. 
If all of the cercariae were thus trapped, one 
could cite this as an example of incompatibility 
due to prevention of escape. It is of interest to 
note that if a similar phenomenon is not 
found in other mollusc-trematode associations, 
one might consider B. glabrata as being partially 
incompatible with S. mansoni as far as escape 
is concerned. Indeed, available information in- 
dicating the incompatibility of different strains 
of B. glabrata with strains of S. mansoni does 
suggest that this relationship is not completely 
free of factors favoring incompatibility. 
SUMMARY 
Compatibility between miracidia and molluscs 
may be determined prior to actual contact. In 
some instances this may be governed by the 
occurrence of host-specific "host factors” which 
stimulate and attract the parasite to a com- 
patible host only. There is sufficient evidence to 
indicate, however, that the attractant need not 
be host-specific and need not indicate subse- 
quent successful establishment and growth. 
Furthermore, ambient environmental conditions 
as well as innate taxes, when such occur, are 
generally stronger determinants of miracidial 
migration and behavior than are chemotactic 
forces. Thus, it is only when the other factors 
serve to bring miracidia into the immediate 
vicinity of the mollusc that chemotaxis becomes 
an effective attractant. There is also evidence 
which suggests that materials extruded from 
molluscs may inhibit rather than serve to en- 
hance host-parasite contact. 
In the case of miracidia which do not hatch 
until the eggs are ingested by the mollusc, the 
physical and chemical factors present in the 
host’s gut can serve as determinants of com- 
patibility or incompatibility. 
It is possible that, during the invasion process, 
both the specificity of the miracidium’s lytic 
secretions and the specificity of substances which 
stimulate the miracidium to shed its epidermal 
plates, secrete lytic enzymes, and invaginate its 
apical papilla could in some instances determine 
the compatibility of the host, especially if these 
are prerequisites of successful penetration. 
Subsequent to successful invasion, the para- 
site generally has to reach a suitable site within 
the mollusc for further development. If this 
migration is prevented by some tissue (s), such 
as a basement membrane, incompatibility re- 
sults. If such a barrier is absent or is overcome, 
the parasite must still overcome the host’s in- 
ternal defense mechanisms (immunity). From 
the information available, cellular immunity, 
both innate and acquired, occurs in molluscs; 
and it is primarily by the formation of capsules 
