110
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
and C. littorinae causing far less damage than C. emascubns. The
relatively mild injuries caused by C. ubiguita and C. littorinae are
believed to be primarily caused by starvation resulting from competition
with the parasites. I n snails infected by the former, he has reported
that the starved condition is reflected in the smaller size of the eggs.
In addition to starvation, toxicity from the parasites’ waste products
and mechanical pressure also contribute to the destruction and atrophy
of some of the germinal cells.
In the case of C. emascuZans, comparatively more severe atrophy
occurs. This is believed to have resulted from the less motile sporocysts which form a “ blocking layer ” at the proximal end of the lymph
channels (blood vessels) supplying the gonad. The “ blocking layer )’
is believed to prevent nutrients from reaching the gonadal tissues and
hence these starve and atrophy. In addition, waste products from the
parasites are believed to contribute to the rapid disintegration of the
host’s gonadal cells.
In the cases of C. emmculans, C. ubiquita and C. littorinae, the hosts’
hepatopancreas is the primary target tissue although €3. leptosoma and
C. lophocerca, both of which develop in rediae, also invade the hepatopancreas secondarily.
In the mollusc’s hepatopancreas, the size and motility of the sporocysts or rediae have been reported to be important factors which govern
the amount of damage inflicted. Since rediae are generally motile,
those of H . leptosoma and C. Zophocerca become evenly distributed
throughout the coiled gland. Of these two species, H . Eeptosoma causes
more damage. W. J. Rees has expressed the opinion that the atrophy
and lysis of the host’s hepatopancreatic cells, particularly the secretory
cells, are due to a combination of starvation resulting from blockage
of the intertubular blood channels and the accumulation of toxic waste
products. Thus physiological factors, rather than direct mechanical
damage, are believed to be more important. In the case of C. lophocerca,
no serious blocking of blood channels occurs. Moreover, no increased
mechanical pressures are found. As the result, Rees has stated that
the host is more able to cope with the excess waste products accumulated by the rediae. Cytologically, his findings are comparable to those
reported by F. G. Rees (1934), including regeneration of the luminal
borders of secretory cells which had been partially destroyed. It is of
interest to note that W. J. Rees has stated that : ‘‘ Snails parasitized by
this cercaria (C. Zophocerca) probably recover from the infestation, as
the efiects produced are not very dangerous as compared with C.
cmasculans.”
Of the three species of larval trematodes which develop in sporo-
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
and C. littorinae causing far less damage than C. emascubns. The
relatively mild injuries caused by C. ubiguita and C. littorinae are
believed to be primarily caused by starvation resulting from competition
with the parasites. I n snails infected by the former, he has reported
that the starved condition is reflected in the smaller size of the eggs.
In addition to starvation, toxicity from the parasites’ waste products
and mechanical pressure also contribute to the destruction and atrophy
of some of the germinal cells.
In the case of C. emascuZans, comparatively more severe atrophy
occurs. This is believed to have resulted from the less motile sporocysts which form a “ blocking layer ” at the proximal end of the lymph
channels (blood vessels) supplying the gonad. The “ blocking layer )’
is believed to prevent nutrients from reaching the gonadal tissues and
hence these starve and atrophy. In addition, waste products from the
parasites are believed to contribute to the rapid disintegration of the
host’s gonadal cells.
In the cases of C. emmculans, C. ubiquita and C. littorinae, the hosts’
hepatopancreas is the primary target tissue although €3. leptosoma and
C. lophocerca, both of which develop in rediae, also invade the hepatopancreas secondarily.
In the mollusc’s hepatopancreas, the size and motility of the sporocysts or rediae have been reported to be important factors which govern
the amount of damage inflicted. Since rediae are generally motile,
those of H . leptosoma and C. Zophocerca become evenly distributed
throughout the coiled gland. Of these two species, H . Eeptosoma causes
more damage. W. J. Rees has expressed the opinion that the atrophy
and lysis of the host’s hepatopancreatic cells, particularly the secretory
cells, are due to a combination of starvation resulting from blockage
of the intertubular blood channels and the accumulation of toxic waste
products. Thus physiological factors, rather than direct mechanical
damage, are believed to be more important. In the case of C. lophocerca,
no serious blocking of blood channels occurs. Moreover, no increased
mechanical pressures are found. As the result, Rees has stated that
the host is more able to cope with the excess waste products accumulated by the rediae. Cytologically, his findings are comparable to those
reported by F. G. Rees (1934), including regeneration of the luminal
borders of secretory cells which had been partially destroyed. It is of
interest to note that W. J. Rees has stated that : ‘‘ Snails parasitized by
this cercaria (C. Zophocerca) probably recover from the infestation, as
the efiects produced are not very dangerous as compared with C.
cmasculans.”
Of the three species of larval trematodes which develop in sporo-