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PACIFIC SCIENCE, Vol. XXII, April 1968 
occurrence of acquired humoral immunity. 
Chowaniec (1961) reported that only a small 
proportion of snails already harboring Fasciola 
hepatica could be infected with the same para- 
site, while most of the control snails could be 
readily infected. In the second study, Lie et al. 
(1966) demonstrated that only 5% of Lymnaea 
ruhiginosa infected with one species of echino- 
stome could be superinfected with a second, 
while 89% of uninfected control snails could 
be infected. In neither of these studies, however, 
were antibodies demonstrated. It is of interest 
that, in the case of L. ruhiginosa, Lie et al. 
stated another possible explanation: young in- 
vading sporocysts of the second species are 
killed and ingested by the rediae of the first 
species. 
The most convincing evidence of acquired 
humoral immunity in molluscs is that contrib- 
uted by Michelson (1963, 1964), who dem- 
onstrated that Schistosoma mansoni miracidia- 
immobilizing substances occur in the tissue ex- 
tracts of Biomphalaria glabrata infected with 
this trematode. Although Michelson found that 
his controls (extracts of other species of un- 
infected snails, extracts of snails infected with 
an acid-fast bacillus, an echinostome metacer- 
caria, the nematode Daubaylia potomaca, snails 
inoculated with bovine albumin, S. mansoni 
eggs, and polystyrene spheres, uninfected B. 
glabrata, and water) all gave positive results, in 
no instance did the percentage of immobiliza- 
tion reach the level observed in extracts of S. 
mansoni- infected snails. Michelson cautiously 
states: "Although the suggestion that the im- 
mobilizing phenomenon might be associated 
with an antigen-antibody interaction is an ap- 
pealing one, data are lacking to substantiate this 
hypothesis. The possibility that the immobilizing 
substance (s) might be related either to parasite- 
produced toxins or to products resulting from 
alternations in the snail’s metabolism cannot be 
excluded.” 
It may be concluded, then, that innate cellu- 
lar immunity appears to be the most efficient 
mechanism by which trematodes are prevented 
from developing in incompatible molluscs, al- 
though acquired cellular immunity may occur. 
The role of humoral factors, either innate or 
acquired, remains uncertain. 
Influence of Host-Elaborated Growth- and 
Development-Stimulating or -Inhibiting 
Substances 
This vast area of host-parasite relationships 
has hardly been touched. From what is known 
about the metabolic interaction between larval 
trematodes and molluscs, it is inconceivable that 
compatible hosts do not influence in some man- 
ner the growth and differentiation processes of 
their parasites and thus enhance their normal 
sequence of development, or, conversely, that in- 
compatible hosts do not in some manner inhibit 
the normal developmental sequences of their 
parasites. 
Meade and Pratt (1966) have reported that, 
when rediae of Metagonimoides oregonensis are 
experimentally transplanted from naturally in- 
fected Oxytrema silicula, in which the gonads 
had been destroyed, to young uninfected snails 
with healthy gonads, a certain number will 
survive but that differences are apparent be- 
tween the transplanted rediae, their progeny, 
and those in naturally infected snails. They 
noted that the transplanted rediae more than 
doubled their natural size, "mucus and debris” 
were included in their caeca, and the enclosed 
metacercariae were no longer distinguishable. 
Burns and Pratt (1953) had shown earlier that 
the rediae of M. oregonensis give rise to both 
cercariae and metacercariae within their brood 
chambers and that no daughter rediae occur. 
Furthermore, although some metacercariae, re- 
leased from transplanted rediae into the body 
cavities of acceptor snails, survived for up to 6 
weeks, none of these were infective when fed 
to a known compatible experimental definitive 
host, the golden hamster. These uninfective 
metacercariae also exhibited some behavioral and 
morphological peculiarities. They displayed 
greater activity, their eyespots disappeared, and 
the prominent Y-shaped excretory vesicle which 
normally appears black was often enlarged and 
possessed fewer granules. Meade and Pratt are 
of the opinion that these differences in the 
transplanted rediae and metacercariae had re- 
sulted from the influence of their new host’s 
gonadal hormone(s). The same hormone(s) 
presumably was present at a very much lower 
level or not present at all in the original cas- 
trated hosts. Whether this conclusion is justified 
must await more direct evidence. 
