4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
25
other workers have presented supporting evidences. Briefly, Mathias
(1925), working with the miracidia of Strigea tarda (= Cotylurus
cornutus), has reported that these demonstrate a preference for Lymnaea
stagnalis but that development would occur in both L. limosa and
L. palustris. Wesenberg-Lund (1934) was convinced by his field
observations that the miracidia of a species of trematode demonstrate
a pronounced preference for a distinot species of mollusc within a given
locality. Neuhaus (1941), who observed the behavior of Fasciola
hepatica miracidia, has suggested that these ciliated larvae are initially
attracted by the ciliary currents maintained by the epithelia of Lymnaea
spp., but later, when they become drawn within a certain range of the
gastropod, the attraction is converted to one purely chemical in nature,
with the effective range varying with the species of Lymnaea used. In
an extension of his earlier observations, Neuhaus (1953) reported a
definite chemotaxis between various species of Lymnaea and F .
hepatica miracidia, with the attraction being most strong with Lymnaea
trunculata, the species generally accepted as the normal host in
Europe (see Kendall, 1950). I n the same paper, Neuhaus stated that
Wirniewski’s observations on Parafasciolopsis fasciolaemorpha miracidia
in the presence of its snail host also suggests chemotaxis.
I n more recent years, several investigators have designed and carried
out more elaborate experiments to prove or disprove the occurrence of
attraction between trematode miracidia and molluscs. Those whose
results favored the “ attraction theory ” are reviewed at this point.
Kloetzel (1958) has carried out a series of carefully controlled
experiments with Schistosoma mansoni miracidia and the snail Australorbis glabratus. In the initial experiment he placed a single snail in
a dish containing a known number of miracidia. He made counts of
the number of miracidia in the immediate vicinity of the snail and a t
other points in the dish at known time intervals. Thus he was able to
demonstrate that the number of miracidia around the snail was significantly higher than at a point diametrically opposed to it after 15 min.
In the second series of experiments he removed the snail from the
miracidia-containing dish after 15 min and, after washing it to remove
adhering larvae, replaced it at the opposite side of the dish for an
additional 15 min. The difference in larval densities at the snail’s
original position and where it was replaced was no longer so significant.
This suggests that some substance was left behind at the initial site
which continued to attract miracidia. Subsequently, Kloetzel has
found that miracidia are even more strongly attracted to a snail
squashed on filter paper than to a living snail and that their attraction
to an empty shell is not significantly more than random. These findings
25
other workers have presented supporting evidences. Briefly, Mathias
(1925), working with the miracidia of Strigea tarda (= Cotylurus
cornutus), has reported that these demonstrate a preference for Lymnaea
stagnalis but that development would occur in both L. limosa and
L. palustris. Wesenberg-Lund (1934) was convinced by his field
observations that the miracidia of a species of trematode demonstrate
a pronounced preference for a distinot species of mollusc within a given
locality. Neuhaus (1941), who observed the behavior of Fasciola
hepatica miracidia, has suggested that these ciliated larvae are initially
attracted by the ciliary currents maintained by the epithelia of Lymnaea
spp., but later, when they become drawn within a certain range of the
gastropod, the attraction is converted to one purely chemical in nature,
with the effective range varying with the species of Lymnaea used. In
an extension of his earlier observations, Neuhaus (1953) reported a
definite chemotaxis between various species of Lymnaea and F .
hepatica miracidia, with the attraction being most strong with Lymnaea
trunculata, the species generally accepted as the normal host in
Europe (see Kendall, 1950). I n the same paper, Neuhaus stated that
Wirniewski’s observations on Parafasciolopsis fasciolaemorpha miracidia
in the presence of its snail host also suggests chemotaxis.
I n more recent years, several investigators have designed and carried
out more elaborate experiments to prove or disprove the occurrence of
attraction between trematode miracidia and molluscs. Those whose
results favored the “ attraction theory ” are reviewed at this point.
Kloetzel (1958) has carried out a series of carefully controlled
experiments with Schistosoma mansoni miracidia and the snail Australorbis glabratus. In the initial experiment he placed a single snail in
a dish containing a known number of miracidia. He made counts of
the number of miracidia in the immediate vicinity of the snail and a t
other points in the dish at known time intervals. Thus he was able to
demonstrate that the number of miracidia around the snail was significantly higher than at a point diametrically opposed to it after 15 min.
In the second series of experiments he removed the snail from the
miracidia-containing dish after 15 min and, after washing it to remove
adhering larvae, replaced it at the opposite side of the dish for an
additional 15 min. The difference in larval densities at the snail’s
original position and where it was replaced was no longer so significant.
This suggests that some substance was left behind at the initial site
which continued to attract miracidia. Subsequently, Kloetzel has
found that miracidia are even more strongly attracted to a snail
squashed on filter paper than to a living snail and that their attraction
to an empty shell is not significantly more than random. These findings