96
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
with great caution. It is well known that even extremely drastic
histopathological changes, such as those produced by pre-adult trematodes in molluscs, need not necessarily indicate impending death. It is
in many ways unfortunate that in the past molluscan pathologists,
especially those interested in the pathology of shellfish, have tended to
equate histopathological change with imminent mortality and thus in
many instances have confused the biological significance and economic
importance of certain host-parasite relationships. Despite many arguments to the contrary, there is still no perfectly satisfactory substitute
for the “ cause and effect ” approach to understanding lethal agents.
In other words, despite the importance of epizootiological evidences,
which may appear to be most convincing, the demonstration of Koch’s
postulates still remains as the one indisputable method of proving
the lethality of a parasite. The story of Hexamita being erroneously
incriminated as a lethal pathogen of oysters a t one time serves as a good
example.
In this section I have chosen not to discuss all aspects of histopathological change in molluscs due to zooparasites. Comments on
changes related to specific parasites are given in Chapters 5-9. I n this
section, only types of changes, together with examples, will be dealt
with.
The position of the parasite is as good a criterion as any to use in
subdividing the following information. For this reason, the subheadings of endoparasitism and ectoparasitism will be employed. The
following outline indicates the subcategories to be considered.
On the shell
In the mantle cavity
Ectoparasitism
Intracellular
Intercellular in tissues
In the alimentary canal
In other organ systems
\ : . * ,
Endoparasitism
--. ,-A’
a. Ectoparasitism
Under this heading are considered those changes associated with
organisms found intimately associated with the outer surfaces of the
shell as well as within the shell but not in the soft parts or in the
alimentary tract.
On the shell. As far as I can determine, no true parasites have been
reported from the exterior of the shells of marine molluscs; in fact,
most of the animals found at these sites have been described as
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
with great caution. It is well known that even extremely drastic
histopathological changes, such as those produced by pre-adult trematodes in molluscs, need not necessarily indicate impending death. It is
in many ways unfortunate that in the past molluscan pathologists,
especially those interested in the pathology of shellfish, have tended to
equate histopathological change with imminent mortality and thus in
many instances have confused the biological significance and economic
importance of certain host-parasite relationships. Despite many arguments to the contrary, there is still no perfectly satisfactory substitute
for the “ cause and effect ” approach to understanding lethal agents.
In other words, despite the importance of epizootiological evidences,
which may appear to be most convincing, the demonstration of Koch’s
postulates still remains as the one indisputable method of proving
the lethality of a parasite. The story of Hexamita being erroneously
incriminated as a lethal pathogen of oysters a t one time serves as a good
example.
In this section I have chosen not to discuss all aspects of histopathological change in molluscs due to zooparasites. Comments on
changes related to specific parasites are given in Chapters 5-9. I n this
section, only types of changes, together with examples, will be dealt
with.
The position of the parasite is as good a criterion as any to use in
subdividing the following information. For this reason, the subheadings of endoparasitism and ectoparasitism will be employed. The
following outline indicates the subcategories to be considered.
On the shell
In the mantle cavity
Ectoparasitism
Intracellular
Intercellular in tissues
In the alimentary canal
In other organ systems
\ : . * ,
Endoparasitism
--. ,-A’
a. Ectoparasitism
Under this heading are considered those changes associated with
organisms found intimately associated with the outer surfaces of the
shell as well as within the shell but not in the soft parts or in the
alimentary tract.
On the shell. As far as I can determine, no true parasites have been
reported from the exterior of the shells of marine molluscs; in fact,
most of the animals found at these sites have been described as