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PACIFIC SCIENCE, Vol. XXII, April 1968 
It should also be mentioned that it is highly 
doubtful if chemotactic forces are influential 
during the pre-invasion relationship between 
miracidia and pelecypod molluscs, since the 
ability of this group of molluscs to effect 
incurrents, through either their siphons or shell 
edges, undoubtedly results in the passive intake 
of miracidia via the currents (Fig. 1). 
Besides innate taxes, the nature of the en- 
vironment is known to determine whether at- 
traction between mollusc and miracidium can 
be effective (Fig. 1). An excellent example of 
this has been contributed by Kawashima et al. 
(196l£). Earlier, these investigators (1961^) 
had demonstrated that, although the miracidium 
of Paragonimus ohirai is attracted to three 
species of brackish water snails of the genus 
Assiminea (A. parasitologic a, A. japonica , and 
A. latericea miyazakii ), only one of the three, 
A. parasitologica, is a compatible host. A. 
latericea miyazakii is an incompatible host, 
while A. japonica can be infected experimen- 
tally but the level of infection is consistently 
low. Thus, it would appear that in nature some 
other factor or factors must be operative to 
bring about the selection of A. parasitologica 
by miracidia. It was subsequently shown, in a 
study of the locomotive speed and survival of 
P. ohirai miracidia in various concentrations of 
NaCl, that the lower the salt concentration is 
the more active the miracidia become, with the 
optimum salinity being 0.25% NaCl or less. 
Concurrent studies on the ecology of the three 
species of snails revealed that the optimum 
salinity for A. parasitologica is 0.25%, that for 
A. latericea miyazakii is 0.4%, and that for A. 
japonica is 0.6%. These findings demonstrate 
that an environmental factor, salinity in this 
case, can serve as a mechanism determining host 
selection. Thus, these investigators have demon- 
trated that the influence of the molluscs’ at- 
tractants can be masked by an ambient factor 
and have also releaved further evidence that 
attraction of miracidia to mollusc need not 
mean successful subsequent development. 
Substances of host origin need not always en- 
hance establishment of the parasite. Some may 
be inhibitory. For example, Cheng et al. 
(1966^) have demonstrated that the plasma of 
seven species of marine pelecypods, Mercenaria 
mercenaria , My a arenaria, Crass ostrea gigas, C. 
virginica, Tapes philippinarum, Mytilus edulis, 
and Modiolus demissus, will stimulate the cer- 
cariae of Himasthla quissetensis to encyst; as 
the result of the rapidity of this process in the 
instances of C. gigas and C. virginica, the cer- 
cariae are prevented from penetrating these 
oysters, especially C. gigas, and hence these 
pelecypods may be considered incompatible 
hosts (Cheng et al., 1966b). These findings 
demonstrate how in some instances a factor of 
host origin operative during the pre-invasion 
phase of host-parasite relationship could deter- 
mine the incompatibility of the host. 
It should be pointed out that the tissue ex- 
tracts of the same seven species of pelecypods 
have a negative effect on H. quissetensis cer- 
cariae, reducing their life spans significantly. 
However, the fact that the cercariae are stimu- 
lated to encyst in M. mercenaria, M. arenaria, 
T . philippinarmn, M. edulis, and M. demissus 
before the tissue component (s) could cause 
their death is believed to be responsible for the 
compatibility of those pelecypods as second 
intermediate hosts. This is because the cyst 
wall serves as a protective layer against con- 
tinued contact with the tissue component (s) . 
Wright (1959) has suggested that the "host 
factor’’ may be in the form of species-specific 
substances incorporated in the body-surface 
mucus of molluscs; Kawashima et al. (1961^) 
have demonstrated that Paragonimus ohirai 
miracidia are attracted to amino acid mixtures 
placed in cellophane bags ; and Maclnnis 
(1965) has found that butyric acid, galactose, 
L-cysteine HC1, and even 1.0 mM HC1 will 
stimulate "contact with return’’ of over 80% of 
S. mansoni miracidia in an artificial test system. 
However, no direct evidence is yet available to 
indicate the nature of "host factors.’’ Moreover, 
it should be pointed out that although organic 
molecules, possibly amino acids, fatty acids, and 
sugars, are the attractants, there is also some 
evidence which indicate that the pH or some 
other physical property of the host may be re- 
sponsible, at least in part, for the attraction 
(Kawashima et al., 1961^) (Fig. 1). This most 
probably explains the attraction of S. mansoni 
miracidia for dilute HC1, as demonstrated by 
Maclnnis. 
In the case of those species of trematodes 
which do not include a free-swimming mira- 
