68
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
that the inner metacercarial cyst wall, secreted by the parasite, is that
agent. The chemical basis remains to be determined.
The concept that the absence or presence of a t least the fibrous
elements (connective tissue fibers and myofibers) of the capsule is
dependent only on their availability in the immediate environment is
not contradictory t o Pan’s (1963) and Tripp’s (1963) belief that leucocytes may come from more distant sources and even that under certain
conditions, fibroblasts and epithelial cells may transform into leucocytes
(Haughton, 1934 ; Wagge, 1951, 1955). Although such transformations
have yet to stand experimental testing, it is possible. However, no one,
up until this time, has convincingly demonstrated the transformation
of phagocytes into fibrous elements in molluscs. As stated, Mackin’s
belief that phagocytes can transform into epithelial cells is in doubt.
Similarly, LabbB’s (1928) report of transformation of amoebocytes to
connective tissue fibers in Doris tuberculafa is in need of confirmation.
In this connection,it is of interest to note that Shrivastava and Richards
(1965), who traced the fate of tritiated thymidine-labelled blood cells
(hemocytes) by autoradiography, have found that blood cells are not
involved in the formation of connective tissue in the wax moth larva,
Galleria mellonella.
Several earlier workers have experimentally induced encapsulation
in molluscs by introducing relatively large foreign materials or through
injury. Drew and De Morgan (1910) have demonstrated this in Pecten
maximus; LabbB (1928, 1929, 1930) has reported this in Doris tuberculata; and Jullien ( 1940) has demonstrated encapsulation in cephalopods.
To emphasize that encapsulation of parasites is known to occur in
so-called natural hosts, although definitely less conspicuously, the
findings of Probert and Erasmus (1965) may be cited. These British
workers have found encapsulation of Cercaria X in the blood vessels
and tissues of the mantle of Lymnaea stagnalis, the natural host of this
trematode.
The observations of Canzonier (personal communication) on the
reactions of the oyster, Crassostrea virginica, to sporocysts of Bucephalus
should also be mentioned. The oyster is the natural intermediate host
for this trematode. Generally no appreciable reactions occur in oysters
parasitized by Bucephalus (see Cheng and Burton, 1965b) ; however,
Canzonier informed me that encapsulation does occur around sporocysts which are moribund or disintegrating*. The question is : did the
* It should be noted that Pauley and Sparks (1965) have reported that Stauber
(personal communication) informed them that : ‘‘ The oyster parasite, Bucephalus sp.,
is not encysted and even when resolution of dead masses of it occurs, it is usually
without encapsulation.”
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
that the inner metacercarial cyst wall, secreted by the parasite, is that
agent. The chemical basis remains to be determined.
The concept that the absence or presence of a t least the fibrous
elements (connective tissue fibers and myofibers) of the capsule is
dependent only on their availability in the immediate environment is
not contradictory t o Pan’s (1963) and Tripp’s (1963) belief that leucocytes may come from more distant sources and even that under certain
conditions, fibroblasts and epithelial cells may transform into leucocytes
(Haughton, 1934 ; Wagge, 1951, 1955). Although such transformations
have yet to stand experimental testing, it is possible. However, no one,
up until this time, has convincingly demonstrated the transformation
of phagocytes into fibrous elements in molluscs. As stated, Mackin’s
belief that phagocytes can transform into epithelial cells is in doubt.
Similarly, LabbB’s (1928) report of transformation of amoebocytes to
connective tissue fibers in Doris tuberculafa is in need of confirmation.
In this connection,it is of interest to note that Shrivastava and Richards
(1965), who traced the fate of tritiated thymidine-labelled blood cells
(hemocytes) by autoradiography, have found that blood cells are not
involved in the formation of connective tissue in the wax moth larva,
Galleria mellonella.
Several earlier workers have experimentally induced encapsulation
in molluscs by introducing relatively large foreign materials or through
injury. Drew and De Morgan (1910) have demonstrated this in Pecten
maximus; LabbB (1928, 1929, 1930) has reported this in Doris tuberculata; and Jullien ( 1940) has demonstrated encapsulation in cephalopods.
To emphasize that encapsulation of parasites is known to occur in
so-called natural hosts, although definitely less conspicuously, the
findings of Probert and Erasmus (1965) may be cited. These British
workers have found encapsulation of Cercaria X in the blood vessels
and tissues of the mantle of Lymnaea stagnalis, the natural host of this
trematode.
The observations of Canzonier (personal communication) on the
reactions of the oyster, Crassostrea virginica, to sporocysts of Bucephalus
should also be mentioned. The oyster is the natural intermediate host
for this trematode. Generally no appreciable reactions occur in oysters
parasitized by Bucephalus (see Cheng and Burton, 1965b) ; however,
Canzonier informed me that encapsulation does occur around sporocysts which are moribund or disintegrating*. The question is : did the
* It should be noted that Pauley and Sparks (1965) have reported that Stauber
(personal communication) informed them that : ‘‘ The oyster parasite, Bucephalus sp.,
is not encysted and even when resolution of dead masses of it occurs, it is usually
without encapsulation.”