4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
109
lodged in the preputium. We believe that some factor or factors
associated with the male reproductive tract, perhaps the pH, discourages the parasites from invading. Thus it would appear that
differences do exist which influence the suitability or unsuitability of
the male reproductive system of molluscs as a site for larval trematode
development. Obviously additional investigations of this nature on
both freshwater and marine gastropods must be forthcoming before
any postulation can be justifiably offered to explain the difference
between the findings of F. G. Rees (1934), W. J. Rees (1936a) and
Cheng and Cooperman (1964).
In addition to invading the hepatopancreas and gonads, larval
trematodes have occasionally been reported from other sites. For
example, F. G. Rees (1934) has found the rediae of Cercaria patellae in
the salivary glands of Patella vulgata, and W. J. Rees (1936b) has
found the sporocysts of Cercaria littorinae in the blood and kidney of
Littorina littorea. Apparently little injury is caused at these sites.
The second extensive study of pathological changes which occur in
a marine snail, Littorina littorea, as the result of parasitization by larval
trematodes has been reported by W. J. Rees (1936a). This investigator
has compared the effects of five species of larval trematodes on the
hepatopancreatic cells and gonads of the molluscan host. The
trematodes studied include Himasthla leptosoma ( = Cercaria himasthh
secunda) which develops in rediae, Cercaria emasculans which develops
in large sporocysts, C. lophocerca which develops in rediae, what may
have been C. ubiquita ( = C. ubiquitoides) although doubtful (but will
be referred to as such in this review) which develops in small sporocysts, and G. littorinae which develops in active sporocysts. He has
concluded from his studies that the amount of pathological change
which occurs in Littorina littorea is dependent upon three major
factors: (1) whether the cercariae develop in sporocysts or rediae;
(2) the sizes of the sporocysts or rediae; and (3) whether inactive
sporocysts form a " blocking layer ".
W. J. Rees has reported that the primary target tissue destroyed or
injured by rediae, and therefore by both Himasthla leptosoma and
Cercaria lophocerca,, is the gonad. It was implied that this is true in all
instances in which the cercariae develop in rediae. Reports since then
have shown this not to be true. Rediae, according to W. J. Rees,
actively ingest gonadal tissues and germinal cells and complete destruction generally occurs. In the case of parasitization by sporocysts, and
therefore by C. emasculans, C. ubiquita and C. littorinae, destruction of
gonadal tissues and gametic cells may also take place. Differences,
however, occur in tho. amount of damage inflicted, with both C. ubiquita
109
lodged in the preputium. We believe that some factor or factors
associated with the male reproductive tract, perhaps the pH, discourages the parasites from invading. Thus it would appear that
differences do exist which influence the suitability or unsuitability of
the male reproductive system of molluscs as a site for larval trematode
development. Obviously additional investigations of this nature on
both freshwater and marine gastropods must be forthcoming before
any postulation can be justifiably offered to explain the difference
between the findings of F. G. Rees (1934), W. J. Rees (1936a) and
Cheng and Cooperman (1964).
In addition to invading the hepatopancreas and gonads, larval
trematodes have occasionally been reported from other sites. For
example, F. G. Rees (1934) has found the rediae of Cercaria patellae in
the salivary glands of Patella vulgata, and W. J. Rees (1936b) has
found the sporocysts of Cercaria littorinae in the blood and kidney of
Littorina littorea. Apparently little injury is caused at these sites.
The second extensive study of pathological changes which occur in
a marine snail, Littorina littorea, as the result of parasitization by larval
trematodes has been reported by W. J. Rees (1936a). This investigator
has compared the effects of five species of larval trematodes on the
hepatopancreatic cells and gonads of the molluscan host. The
trematodes studied include Himasthla leptosoma ( = Cercaria himasthh
secunda) which develops in rediae, Cercaria emasculans which develops
in large sporocysts, C. lophocerca which develops in rediae, what may
have been C. ubiquita ( = C. ubiquitoides) although doubtful (but will
be referred to as such in this review) which develops in small sporocysts, and G. littorinae which develops in active sporocysts. He has
concluded from his studies that the amount of pathological change
which occurs in Littorina littorea is dependent upon three major
factors: (1) whether the cercariae develop in sporocysts or rediae;
(2) the sizes of the sporocysts or rediae; and (3) whether inactive
sporocysts form a " blocking layer ".
W. J. Rees has reported that the primary target tissue destroyed or
injured by rediae, and therefore by both Himasthla leptosoma and
Cercaria lophocerca,, is the gonad. It was implied that this is true in all
instances in which the cercariae develop in rediae. Reports since then
have shown this not to be true. Rediae, according to W. J. Rees,
actively ingest gonadal tissues and germinal cells and complete destruction generally occurs. In the case of parasitization by sporocysts, and
therefore by C. emasculans, C. ubiquita and C. littorinae, destruction of
gonadal tissues and gametic cells may also take place. Differences,
however, occur in tho. amount of damage inflicted, with both C. ubiquita