4. ANALYSIS O F FACTORS INVOLVED I N SYMBIOSIS
63
and gradually are engulfed as the web-like leucocytic ectoplasm flowed
and filled in between the pseudopods. The curious anchoring or sticking
of bacteria to amoebocytes prior to actual phagocytosis is explained
by Bang as the result of a fortunate electromicrograph. He has
demonstrated the unipolar flagellum of a bacterium wrapped around
an amoebocyte’s filamentous pseudopodia. This explains the continual
tugging and jerking at an invisible “ anchor ” when observed under
the light microscope. Thus, at least in this case, the sticking of the
foreign particle (bacterium) to be engulfed to the amoebocyte appears
to be a mechanical process.
It is of interest to note that Bang has found that not all bacteria
are phagocytized. In some instances amoebocytes were observed to
approach bacteria with their filamentous pseudopods but then they
either reversed their flow or turned aside. This process was observed
for several hours and no phagocytosis was observed. The cause of this
unusual behavior pattern remains unknown. Bang stated that “ . . . it
remains likely that there is an undiscovered factor important in phagocytosis which is responsible for this variation.”
The studies cited above, particularly those by Stauber’s group at
Rutgers University, convincingly demonstrate the efficiency of
molluscan phagocytes as an internal defense mechanism. However,
it should be pointed out that a few reports are available which suggest
the ineffectiveness, i.e. non-destruction, of phagocytes against certain
invading organisms. For example, Michelson (1961) has stated that in
planorbid snails, acid-fast bacteria can multiply within phagocytes
and presumably can be carried by them to uninfected tissues. Later
(Michelson, 1963b), he reported the absence of cellular response in
aquatic pulmonate snails parasitized by certain microsporidea. Pan
(1956) has also shown the apparent inability of Australorbis glabratus
phagocytes to cope with yeast-like organisms found in the nerve cells
and amoebocytes of naturally infected hosts, and Mackin (1962) has
reported the lack of cellular reaction in Crassostrea virginica, including
phagocytosis, to a mycelial parasite and to the sporozoan Nematopsis
although in the latter case Feng (1958) has suggested that C. virginica
can rid itself of this parasite. Feng based his conclusion on experiments
in which he transplanted oysters with high and low initial infections to
areas of low and high infections respectively and found that the transplanted oysters attained the characteristic level of infections of native
oysters in that area. He concluded that presumably “ a dynamic
equiIibrium of elimination and reinfection of the parasite was reached.
There is no direct evidence that Nematopsis spores can be eliminated
by phagocytes.
63
and gradually are engulfed as the web-like leucocytic ectoplasm flowed
and filled in between the pseudopods. The curious anchoring or sticking
of bacteria to amoebocytes prior to actual phagocytosis is explained
by Bang as the result of a fortunate electromicrograph. He has
demonstrated the unipolar flagellum of a bacterium wrapped around
an amoebocyte’s filamentous pseudopodia. This explains the continual
tugging and jerking at an invisible “ anchor ” when observed under
the light microscope. Thus, at least in this case, the sticking of the
foreign particle (bacterium) to be engulfed to the amoebocyte appears
to be a mechanical process.
It is of interest to note that Bang has found that not all bacteria
are phagocytized. In some instances amoebocytes were observed to
approach bacteria with their filamentous pseudopods but then they
either reversed their flow or turned aside. This process was observed
for several hours and no phagocytosis was observed. The cause of this
unusual behavior pattern remains unknown. Bang stated that “ . . . it
remains likely that there is an undiscovered factor important in phagocytosis which is responsible for this variation.”
The studies cited above, particularly those by Stauber’s group at
Rutgers University, convincingly demonstrate the efficiency of
molluscan phagocytes as an internal defense mechanism. However,
it should be pointed out that a few reports are available which suggest
the ineffectiveness, i.e. non-destruction, of phagocytes against certain
invading organisms. For example, Michelson (1961) has stated that in
planorbid snails, acid-fast bacteria can multiply within phagocytes
and presumably can be carried by them to uninfected tissues. Later
(Michelson, 1963b), he reported the absence of cellular response in
aquatic pulmonate snails parasitized by certain microsporidea. Pan
(1956) has also shown the apparent inability of Australorbis glabratus
phagocytes to cope with yeast-like organisms found in the nerve cells
and amoebocytes of naturally infected hosts, and Mackin (1962) has
reported the lack of cellular reaction in Crassostrea virginica, including
phagocytosis, to a mycelial parasite and to the sporozoan Nematopsis
although in the latter case Feng (1958) has suggested that C. virginica
can rid itself of this parasite. Feng based his conclusion on experiments
in which he transplanted oysters with high and low initial infections to
areas of low and high infections respectively and found that the transplanted oysters attained the characteristic level of infections of native
oysters in that area. He concluded that presumably “ a dynamic
equiIibrium of elimination and reinfection of the parasite was reached.
There is no direct evidence that Nematopsis spores can be eliminated
by phagocytes.