4. ANALYSIS O F FACTORS INVOLVED IN SYMBIOSIS
39
It seems probable that the total period of incubation of F . magna eggs may
influence the ability of the miracidia to establish contact with their molluscan
hosts. The age of the miracidia at the time of hatching perhaps controls their
ability to respond to the presence of a snail. It is possible that miracidia
which hatch as soon as their development permits, have not yet acquired a
sensitivity to whatever stimulus (chemical or otherwise) the snail may provide. Since miracidia have been observed to exhibit excitement [italics added]
in the presence of a snail, it is felt that under certain conditions, not yet
understood, a marked attraction on the part of a snail for a miracidium
may exist.
Since the age of the parasite does influence the efficiency of contact
with its host, it appears feasible that other factors, such as the age of
the host, may also be influential but this has not yet been tested.
TABLE Iv. DISAPPEARANCE OF Pascioloides magna MIRACIDIA IN THE PRESENCE
OB Stagnicola rejlexa EXPRESSED AS PERCENTAGES
Miracidia of series A were 10-40 rnin post-hatching, those of series B 92-104 min
post-hatching, and those of series C 520-523 min post-hatching. (After Campbell
and Todd, 1955a.)
Time interval
‘‘ Young ”
‘‘ Medium ’’
“ Old ”
(min)
(series A )
(series B )
(aerie8 C )
8-15
54
76
47
-
15-35
86.5
88
35-40
84
82
40-50
50-70
86.5
-
-
75
78
-
-
Campbell and Todd have also compared the percentages of contact
between F. magna miracidia and two species of molluscs. The parasites
used were between 41 and 56 min old. It was noted that :
No difference was detected between the swimming behavior of the miracidium
in the presence of Xtagnicola re$exa and Fossaria modicella rustica but the
results of this experiment suggest that the rniracidia, attack the latter species
more readily.
The significance of this last study, in my opinion, rests with the fact
that the miracidia did become attached to two species of molluscs.
Although both of these happento be compatible hosts (Krull, 1933;
Campbell and Todd, 1955b), it is interesting that both snails “attracted”
miracidia and hence indicate that the “ attraction ” is not speciesspecific.
A large part of this controversy pertaining t o the existence or nonexistence of an attractant of host origin, as I interpret it, is based on
several fallacies.
39
It seems probable that the total period of incubation of F . magna eggs may
influence the ability of the miracidia to establish contact with their molluscan
hosts. The age of the miracidia at the time of hatching perhaps controls their
ability to respond to the presence of a snail. It is possible that miracidia
which hatch as soon as their development permits, have not yet acquired a
sensitivity to whatever stimulus (chemical or otherwise) the snail may provide. Since miracidia have been observed to exhibit excitement [italics added]
in the presence of a snail, it is felt that under certain conditions, not yet
understood, a marked attraction on the part of a snail for a miracidium
may exist.
Since the age of the parasite does influence the efficiency of contact
with its host, it appears feasible that other factors, such as the age of
the host, may also be influential but this has not yet been tested.
TABLE Iv. DISAPPEARANCE OF Pascioloides magna MIRACIDIA IN THE PRESENCE
OB Stagnicola rejlexa EXPRESSED AS PERCENTAGES
Miracidia of series A were 10-40 rnin post-hatching, those of series B 92-104 min
post-hatching, and those of series C 520-523 min post-hatching. (After Campbell
and Todd, 1955a.)
Time interval
‘‘ Young ”
‘‘ Medium ’’
“ Old ”
(min)
(series A )
(series B )
(aerie8 C )
8-15
54
76
47
-
15-35
86.5
88
35-40
84
82
40-50
50-70
86.5
-
-
75
78
-
-
Campbell and Todd have also compared the percentages of contact
between F. magna miracidia and two species of molluscs. The parasites
used were between 41 and 56 min old. It was noted that :
No difference was detected between the swimming behavior of the miracidium
in the presence of Xtagnicola re$exa and Fossaria modicella rustica but the
results of this experiment suggest that the rniracidia, attack the latter species
more readily.
The significance of this last study, in my opinion, rests with the fact
that the miracidia did become attached to two species of molluscs.
Although both of these happento be compatible hosts (Krull, 1933;
Campbell and Todd, 1955b), it is interesting that both snails “attracted”
miracidia and hence indicate that the “ attraction ” is not speciesspecific.
A large part of this controversy pertaining t o the existence or nonexistence of an attractant of host origin, as I interpret it, is based on
several fallacies.