28
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
chamber ( 3 x 1.5 cm diam.). The inner surface of the maze was
coated with inert waterproof lacquer to prevent contamination and to
facilitate cleaning between trials. In the first series of experiments,
two specimens of Australorbis glabratus, the shells of which had been
crushed, were placed in two of the terminal chambers, one in each. In
each of the other two terminal chambers was placed a sham snail
modelled out of inert aquarium cement. About 100-200 Xchistosoma
mansoni miracidia were placed in the central chamber in conditioned
water after the crushed and sham snails had been permitted to stand
in the terminal chambers for 1 h, during which time substances of
crushed snail origin had entered the water. After the central chamber
was covered, the entire apparatus was placed under a strong light
source to prevent miracidia which had reached any of the terminal
chambers from returning to the central chamber. This was carried out
since it is known that certain schistosome miracidia are positively
phototactic. Etges and Decker were careful to state that with the
sham snails in pIace, the amounts of light reflected into the central
chamber through the four arms were essentially equal. After 1 h 80 min,
80% of the miracidia had entered the side arms which were then
stoppered at the center chamber-arm junctions. Counts of the number
of miracidia in each of the terminal chambers and adjoining arms
during nine runs revealed that the number of miracidia in the terminal
chambers and arms associated with crushed snails was significantly
greater. As a result, these authors stated: “Such a great degree of
significance strongly indicates positive chemotaxis of S. rnansoni
miracidia toward A. glabratus under these experimental conditions.”
Since it was observed that crushed A . glabratus gave a slight reddish
turbidity to the water resulting from released hemoglobin, thus
decreasing the amount of light transmitted to the center chamber
from the two arms leading t o the real snails, and it was feared that
some of the miracidia which had reached the crushed snails had entered
the hosts’ tissues and had thus been missed in the counting, a second
series of experiments, involving uncrushed snails restrained by loosely
wrapped nylon mesh, was conducted. Again, the live snails attracted
significantly more miracidia. In addition to using A. glabratus, two
other groups of gastropods, Helisoma anceps and a mixture of Bulinus
(Bulinus) truncatus and B. (Physopsis) sp., both crushed, were employed in identical experiments. The results revealed that the miracidia
were distributed in all cases in favor of the sham snails. Rather than
interpreting this to mean that repulsion occurred between Bulinus spp.
or H . anceps and the miracidia, the authors offered the explanation
that the condition resulted from less transmitted light from the arms
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
chamber ( 3 x 1.5 cm diam.). The inner surface of the maze was
coated with inert waterproof lacquer to prevent contamination and to
facilitate cleaning between trials. In the first series of experiments,
two specimens of Australorbis glabratus, the shells of which had been
crushed, were placed in two of the terminal chambers, one in each. In
each of the other two terminal chambers was placed a sham snail
modelled out of inert aquarium cement. About 100-200 Xchistosoma
mansoni miracidia were placed in the central chamber in conditioned
water after the crushed and sham snails had been permitted to stand
in the terminal chambers for 1 h, during which time substances of
crushed snail origin had entered the water. After the central chamber
was covered, the entire apparatus was placed under a strong light
source to prevent miracidia which had reached any of the terminal
chambers from returning to the central chamber. This was carried out
since it is known that certain schistosome miracidia are positively
phototactic. Etges and Decker were careful to state that with the
sham snails in pIace, the amounts of light reflected into the central
chamber through the four arms were essentially equal. After 1 h 80 min,
80% of the miracidia had entered the side arms which were then
stoppered at the center chamber-arm junctions. Counts of the number
of miracidia in each of the terminal chambers and adjoining arms
during nine runs revealed that the number of miracidia in the terminal
chambers and arms associated with crushed snails was significantly
greater. As a result, these authors stated: “Such a great degree of
significance strongly indicates positive chemotaxis of S. rnansoni
miracidia toward A. glabratus under these experimental conditions.”
Since it was observed that crushed A . glabratus gave a slight reddish
turbidity to the water resulting from released hemoglobin, thus
decreasing the amount of light transmitted to the center chamber
from the two arms leading t o the real snails, and it was feared that
some of the miracidia which had reached the crushed snails had entered
the hosts’ tissues and had thus been missed in the counting, a second
series of experiments, involving uncrushed snails restrained by loosely
wrapped nylon mesh, was conducted. Again, the live snails attracted
significantly more miracidia. In addition to using A. glabratus, two
other groups of gastropods, Helisoma anceps and a mixture of Bulinus
(Bulinus) truncatus and B. (Physopsis) sp., both crushed, were employed in identical experiments. The results revealed that the miracidia
were distributed in all cases in favor of the sham snails. Rather than
interpreting this to mean that repulsion occurred between Bulinus spp.
or H . anceps and the miracidia, the authors offered the explanation
that the condition resulted from less transmitted light from the arms