Compatibility in Trematodes and Molluscs — Cheng 
147 
cidial stage, successful host-parasite contact is 
dependent upon the ingestion of the egg by the 
mollusc, followed by hatching within the latter’s 
alimentary tract (Fig. 1). Although some infor- 
mation is available pertaining to the hatching 
process of eggs in water (Standen, 1951; 
Rowan, 1956, 1957), surprisingly little infor- 
mation is available on the factor or factors which 
influence hatching of trematode eggs in mol- 
luscs. It is generally believed that the mollusc’s 
digestive juices in some manner stimulate hatch- 
ing (Krull and Mapes, 1952; Timon-David, 
1965; and others), although the exact mech- 
anisms have not been studied. Nevertheless, it 
is apparent that the biochemistry and physical 
properties of the mollusc’s digestive tract could 
serve as determinants of compatibility, as mani- 
fested by hatching, or of incompatibility, as 
manifested by nonhatching (Fig. 1). 
THE INVASION PROCESS 
Because of obvious medical implications, con- 
siderable information is available about factors 
governing the penetration of cercariae into 
mammals and other vertebrate hosts. On the 
other hand, little is known about miracidial 
penetration into molluscs, either from the ex- 
terior or through the gut wall. Nevertheless, 
this barrier could be a factor determining com- 
patibility or incompatibility. 
Certain aspects of the processes involved dur- 
ing the successful penetration of Lymijaea 
(= Limnaea') truncatula and L. auricularia by 
Fasciola hepatica and F. gigantica, respectively, 
have been studied by Dawes (1959, 196 O 4 , b, 
c) . According to him, the miracidium first 
becomes attached to the molluscan host’s integu- 
ment by suctorial action resulting from applica- 
tion of the "cup” formed by the inversion of 
the anterior papilla assisted by mucus. This is 
followed by the secretion of cytolytic enzymes 
from "... the gut and the unicellular pharyn- 
geal 'glands’ into the 'cup.’ ” The subsequent 
enzymatic activity results in the lysis of the 
host’s epithelial and subepithelial tissues. Only 
after the host’s integument has been perforated 
does the parasite enter, but not before it has 
sloughed its ciliated epidermis. For this reason, 
! Dawes considers the penetrating form to be a 
| sporocyst and not a miracidium. 
The question may be asked whether the mira- 
cidial cytolytic enzymes must be chemically spe- 
cific for the integument of specific species of 
molluscs. If this is the case, the compatibility 
of enzyme to substrate could serve as a factor 
governing successful penetration, hence host- 
compatibility (Fig. 2). Indeed, Lie (1963) be- 
lieves that the prevention of penetration of 
certain echinostome miracidia by unnatural snails 
is responsible for incompatibility. 
In addition to the miracidium’s cytolytic en- 
zymes, Dawes (1960c) has expressed the view 
that the shedding of the ciliated epidermis by 
the miracidium, thus transforming it to a sporo- 
cyst, is a prerequisite for successful infection of 
the snail. This hypothesis has been challenged 
by Lengy (1962), who has demonstrated that 
Schistosoma bovis miracidia do not shed their 
ciliated epidermis prior to penetrating. Similarly, 
Maldonado and Metienzo (1947) demonstrated 
earlier that S. mansoni miracidia do not shed 
their plates until after successful penetration ; 
and Heyneman (1966) has successfully initiated 
the infection of Lymnaea rubiginosa with Echi- 
nostoma audyi miracidia and of Indoplanorbis 
exustus with Echino stoma malayanum miracidia 
by inoculating these miracidia through the 
mantle via a minute hole drilled in the molluscs’ 
shells, thus suggesting that the shedding of the 
miracidial epidermal plates is not a prerequisite 
to successful infection, at least in these species. 
It remains true, nevertheless, that certain species 
of fasciolid miracidia may shed their plates 
prior to penetration. Campbell and Todd 
► Stimulation to shed epidermal plates (factor(s) in plasma) 
► Stimulation to invaginate apical papilla 
► Stimulation to secrete cytolytic enzyme 
► Specificity of cytolytic enzyme (?) 
Fig. 2. Diagrammatic drawing illustrating factors 
which may govern or influence successful penetration 
of mollusc by miracidium. 
