148 
PACIFIC SCIENCE, Vol. XXII, April 1968 
(1955), for example, have reported that Fascio- 
loides magna miracidia shed their ciliated 
plates on the exterior after a brief contact with 
the molluscan host’s tissues, and Barlow (1925) 
has found the transformation of F. buski mira- 
cidia into sporocysts when bathed in "snail tissue 
juices.” I observed that F. gigantica miracidia 
shed their plates when placed in concentrated 
and 1:10 dilutions of the plasma of Galba 
ollula, the natural host (Fig. 3), and in similar 
concentrations of the plasma of Helisoma duryi 
normale, an incompatible host, but not in the 
plasma of Tarebia granifera mauiensis and Lit- 
torina pintado, both of which are also incom- 
patible hosts. Similar phenomena were not 
observed when miracidia were placed in tissue 
extracts of all four species of snails, nor were 
they observed in greater dilutions of G. ollula 
and H. duryi normale plasma. Thus, this phe- 
nomenon is apparently not related to host com- 
patibility. Rather it suggests that miracidia 
which possess the innate ability to shed their 
epidermal plates prior to penetration can be 
stimulated to do so by some factor(s). Never- 
theless, it would appear that the occurrence of 
the stimulatory factor (s) in the natural hosts 
could influence compatibility during this phase 
of host-parasite relationship, especially if Dawes’ 
contention is true among fasciolid trematodes 
(Fig. 2). 
No information is yet available relative to 
the nature of the stimulatory factor(s); how- 
ever, the fact that F. gigantica miracidia did not 
shed their plates when placed in extracts of 
desanguinated and aqueously perfused snails 
indicates that the factor is present in plasma 
rather than in tissue fluids. 
It is also significant that F. gigantica miracidia 
were stimulated to shed their plates only in con- 
centrated plasma and in a 1:10 dilution of 
plasma. This may be interpreted to mean that 
in nature the stimulatory effect would occur only 
when miracidia become intimately associated 
with or are in actual contact with the snail, 
since plasma, seeping from the wound resulting 
from the parasite’s lytic enzymes, would be 
rapidly diluted as it diffuses though the aqueous 
medium. 
In addition to the shedding of epidermal 
plates, invagination of the apical papilla as well 
as the secretion of some substance, perhaps the 
lytic enzyme, were also noticed in F. gigantica 
miracidia exposed to plasma from G. ollula and 
H. duryi normale (Fig. 4). Thus it would ap- 
pear that the formation of the apical "cup” as 
well as secretion are stimulated not by physical 
contact but by some factor (s) present in the 
mollusc’s plasma (Fig. 2). 
ESTABLISHMENT OF THE PARASITE 
Successful establishment of germinal sacs 
(sporocysts, rediae, or both) within the mollusc 
implies that the form which has invaded the 
host will reach a suitable site, overcome the 
host’s internal defense mechanisms, be the target 
of host-elaborated growth and development- 
stimulating factors, be able to obtain its required 
nutrients, and at the same time not kill its 
host (Fig. 5). These requirements are con- 
sidered separately. 
Reaching a Suitable Site 
Although tissue specificity still remains one 
of the unsolved problems in parasitology, it is a 
well-documented phenomenon. For example, for 
a large number of species among the Digenea 
the molluscan host’s hepatopancreas or gonads 
appear to be the preferred sites of normal larval 
development. This does not mean that aberrant 
parasites cannot develop in ectopic sites. Indeed, 
such exceptions to the rule are known. Investi- 
gations into the nature and development of 
larvae which grow at ectopic sites not only can 
provide insights into the physico-chemical re- 
quirements of these parasites but also can reveal 
some of the factors which inhibit or prevent the 
parasite from reaching its normal developmental 
site. A series of such studies is summarized to 
illustrate this point. 
Among the Plagiorchioidea, the mother spor- 
ocysts of certain species are known to be at- 
tached to the external surface of their molluscan 
hosts’ alimentary tracts (Cort et ah, 1954; 
Rankin, 1944; Leigh, 1946; Schell, 1961, 
1962^/ Cheng, 1961^, b ; and others). Sur- 
rounding each of the daughter sporocysts arising 
from these mother sporocysts is a so-called 
paletot. The question is: what is the origin of 
this paletot? According to Cort and Olivier 
(1943), Cort and Ameel (19 44), and Cort et 
al. (1954), the paletot is formed from the 
