4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
77
The belief that antibodies do occur in molluscs stems primarily from
the studies of Winfield (1932) and Nolf and Cort (1933) who have
indirectly suggested that acquired immunity may be present in gastropod intermediate hosts of larval trematodes. They reported that the
presence of sporocysts of Cotylurus JEabelliformis, enclosing developing
cercariae, in varieties of Lymnaea stagnalis prevents almost all of the
cercariae of this trematode from successfully penetrating and encysting
as metacercariae, even when the snails are exposed to large numbers
of cercariae. Cort et al. (1945) have repeated their studies and have
reported that the same phenomenon occurs in Stagnicola emarginata
angulata infected with Cotylurus Jlabelliformis sporocysts They did add
that the few cercariae which do succeed in penetrating are inhibited
from developing into metacercariae unless they enter sporocysts and
are thus presumably protected from the host’s antibodies. On the basis
of these reports, Culbertson (1941) has generalized that “ . . . it is
clear that snails acquire an immunity after infection by trematodes.”
This sweeping statement is premature since, as stated, antibodies in
molluscs have not yet been demonstrated.
Other indirect pieces of evidence of acquired immunity in molluscs
exist. Dye (1924), for example, has observed that if the snail Melania
nodocinca in Africa is naturally infected with a ‘‘ single-tailed cercariae”, infection with Schistosorna haematobiurn cannot be superimposed. Gordon et al. (1934) have considered Dye’s results to indicate
the occurrence of acquired immunity ; however, Michelson (1963a,
1964) has correctly pointed out that Dye’s observations should be
accepted with reservation since it has since been shown that only one
species of Bulinus serves as the intermediate host for S. haematobiurn
in Africa.
Benex and Lamy (1959) discovered that tissue extracts from the
planorbid snail Planorbis corneus (which is not an intermediate host
for mammalian schistosomes) will immobilize Schistosonza mansoni
miracidia. Based on this observation, these French workers have
suggested that species of snails found refractory to schistosome
infection may possess “ immune-like ” immobilizing substances. It is
extremely difficult, if not impossible, to evaluate Benex and Lamy’s
results since, although Planorbis corneus is not a known host for
mammalian schistosomes, it can serve as a host for non-mammalian
schistosomes. Thus, it cannot be determined whether the “ immunelike ” immobilizing substances are acquired as the reault of previous
experience with non-mammalian schistosomes which share common
antigens with mammalian schistosomes, or whether it is an innate
humoral factor similar to the cercaricidal substance found by Cheng
77
The belief that antibodies do occur in molluscs stems primarily from
the studies of Winfield (1932) and Nolf and Cort (1933) who have
indirectly suggested that acquired immunity may be present in gastropod intermediate hosts of larval trematodes. They reported that the
presence of sporocysts of Cotylurus JEabelliformis, enclosing developing
cercariae, in varieties of Lymnaea stagnalis prevents almost all of the
cercariae of this trematode from successfully penetrating and encysting
as metacercariae, even when the snails are exposed to large numbers
of cercariae. Cort et al. (1945) have repeated their studies and have
reported that the same phenomenon occurs in Stagnicola emarginata
angulata infected with Cotylurus Jlabelliformis sporocysts They did add
that the few cercariae which do succeed in penetrating are inhibited
from developing into metacercariae unless they enter sporocysts and
are thus presumably protected from the host’s antibodies. On the basis
of these reports, Culbertson (1941) has generalized that “ . . . it is
clear that snails acquire an immunity after infection by trematodes.”
This sweeping statement is premature since, as stated, antibodies in
molluscs have not yet been demonstrated.
Other indirect pieces of evidence of acquired immunity in molluscs
exist. Dye (1924), for example, has observed that if the snail Melania
nodocinca in Africa is naturally infected with a ‘‘ single-tailed cercariae”, infection with Schistosorna haematobiurn cannot be superimposed. Gordon et al. (1934) have considered Dye’s results to indicate
the occurrence of acquired immunity ; however, Michelson (1963a,
1964) has correctly pointed out that Dye’s observations should be
accepted with reservation since it has since been shown that only one
species of Bulinus serves as the intermediate host for S. haematobiurn
in Africa.
Benex and Lamy (1959) discovered that tissue extracts from the
planorbid snail Planorbis corneus (which is not an intermediate host
for mammalian schistosomes) will immobilize Schistosonza mansoni
miracidia. Based on this observation, these French workers have
suggested that species of snails found refractory to schistosome
infection may possess “ immune-like ” immobilizing substances. It is
extremely difficult, if not impossible, to evaluate Benex and Lamy’s
results since, although Planorbis corneus is not a known host for
mammalian schistosomes, it can serve as a host for non-mammalian
schistosomes. Thus, it cannot be determined whether the “ immunelike ” immobilizing substances are acquired as the reault of previous
experience with non-mammalian schistosomes which share common
antigens with mammalian schistosomes, or whether it is an innate
humoral factor similar to the cercaricidal substance found by Cheng