36
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
to be aware of the presence of the snails, even when contact was made
(Sudd’s type 1 behavior pattern). Of the seven normal host-parasite
combinations, all demonstrated either type 3 or type 4 behavior
patterns. From these studies, Sudds has concluded that :
It is clear, therefore, that under the controlled conditions of this study there
is little support for the “ attraction theory ”, since only 2 of the 19 combinations gave results of the type expected under the operation of a specific
chemical stimulus. On the other hand, 17 of the 19 combinations yielded
results supporting the view that contact with snail hosts occurs by chance,
and that, once contact is made, the miracidia are stimulated to attempt
penetration.
Although Sudds’ subsequent histopathological studies on the
reaction of compatible and incompatible snails to schistosomes are to
be commended, it is not at all clear that his “ attraction” studies
proved what he concluded. The reasons for this doubt are discussed
below.
Chernin and Dunavan (1962), in discussing a series of experiments
designed to test the influence of host-parasite dispersion on the capacity
of Schistosoma mansoni miracidia to infect Australorbis glabratus,
stated that they found nothing to support the “ attraction theory ”.
They further stated that:
. . . nor do the experiments and rationalizations which have been published
in support of it (the attraction theory) seem entirely convincing. This is
not to say that ‘‘ attraction ” between snail and schistosome miracidium
does not exist, but rather that the fact of its existence has not yet been
demonstrated unequivocally.
I n critically analyzing the data by Chernin and Dunavan, I failed to see
how their experiments in any manner either support or deny the
existence of a chemotactic factor. Their experiments were designed to
determine whether a t a constant water level, regardless of volume, the
capacity of miracidia to infect snails was influenced. They found that
there is no significant difference in the different volumes used. They
also studied the preference of miracidia for different depths and distances in reaching hosts. They found that miracidia do not demonstrate
a preference for any specific depth when placed in 20 cm of water and
that some miracidia traverse a t least 86 cm horizontally or 33 cm
downwards to reach molluscan hosts. Their other experiments demonstrated that miracidia do not follow a linear course and travel a t a
velocity of 2.1 mm per sec (range 1.7-2.8 mm per sec) during the first
15 min after emergence and average 1.9 mm per sec (range 1.3-2.5 mm
per sec) after about 1 h of free life. Their studies on miracidial taxes
disclosed that negative geotaxis has a stronger influence on their
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
to be aware of the presence of the snails, even when contact was made
(Sudd’s type 1 behavior pattern). Of the seven normal host-parasite
combinations, all demonstrated either type 3 or type 4 behavior
patterns. From these studies, Sudds has concluded that :
It is clear, therefore, that under the controlled conditions of this study there
is little support for the “ attraction theory ”, since only 2 of the 19 combinations gave results of the type expected under the operation of a specific
chemical stimulus. On the other hand, 17 of the 19 combinations yielded
results supporting the view that contact with snail hosts occurs by chance,
and that, once contact is made, the miracidia are stimulated to attempt
penetration.
Although Sudds’ subsequent histopathological studies on the
reaction of compatible and incompatible snails to schistosomes are to
be commended, it is not at all clear that his “ attraction” studies
proved what he concluded. The reasons for this doubt are discussed
below.
Chernin and Dunavan (1962), in discussing a series of experiments
designed to test the influence of host-parasite dispersion on the capacity
of Schistosoma mansoni miracidia to infect Australorbis glabratus,
stated that they found nothing to support the “ attraction theory ”.
They further stated that:
. . . nor do the experiments and rationalizations which have been published
in support of it (the attraction theory) seem entirely convincing. This is
not to say that ‘‘ attraction ” between snail and schistosome miracidium
does not exist, but rather that the fact of its existence has not yet been
demonstrated unequivocally.
I n critically analyzing the data by Chernin and Dunavan, I failed to see
how their experiments in any manner either support or deny the
existence of a chemotactic factor. Their experiments were designed to
determine whether a t a constant water level, regardless of volume, the
capacity of miracidia to infect snails was influenced. They found that
there is no significant difference in the different volumes used. They
also studied the preference of miracidia for different depths and distances in reaching hosts. They found that miracidia do not demonstrate
a preference for any specific depth when placed in 20 cm of water and
that some miracidia traverse a t least 86 cm horizontally or 33 cm
downwards to reach molluscan hosts. Their other experiments demonstrated that miracidia do not follow a linear course and travel a t a
velocity of 2.1 mm per sec (range 1.7-2.8 mm per sec) during the first
15 min after emergence and average 1.9 mm per sec (range 1.3-2.5 mm
per sec) after about 1 h of free life. Their studies on miracidial taxes
disclosed that negative geotaxis has a stronger influence on their