48
M. J. WELLS
for example, may move away from a place in which it was given a
series of electric shocks hours later, during the following night. From
the time scale, with any other type of animal, it would be not unreasonable to suppose that the move was a result of association of area
and shock ; the animals found the region inhospitable and moved on.
Yet because anemones respond so slowly, the possibility remains that
the shocks do no more than cause a switch in long-term patterns of
activity, with results that show up hours later as an apparently adaptive reaction to remembered circumstances (Pantin, 1952). The very
variety of the long term activities of the animals means (as with
echinoderms) that it is diBcult to distinguish whether they are selecting
one of a series of possible action patterns as a result of experience, that
is learning, or simply switching to any of the available alternatives
when the first doesn’t work.
Bearing this in mind and without, at this stage, prejudging the
issue by defining what would constitute evidence of learning in a
coelenterate, it is worth examining the experiments that have been
held to demonstrate learning in the past. These are :
1. Cessation of responses to repeated stimuli that at f i s t cause
withdrawal.
2. Cessation of responses to inedible objects (usually bits of
filter paper soaked in meat juice) repeatedly presented.
3. Persistence and gradual fading of tidal and diurnal
rhythms in anemones brought into constant conditions.
4. Position habits in individuals.
The first type of experiment is clearly a study of habituation.
Aiptasia, for example, normally withdraws when it is suddenly stimulated by a drip falling into the still water of its aquarium. If the drip
occurs repeatedly at intervals of a minute or so (long enough for the
anemone to expand its tentacles again) the response fades after two to
three trials. If the drips are more than about 5 min apart, it does not
(Jennings, 1905). Such habituative phenomena must be general ;
anemones moved from a quiet environment to aquaria with a brisk
circulation do not remain permanently withdrawn. What is not known
is whether one is dealing with sensory accommodation, or changes sited
more deeply in the nervous system. Most definitions of learning would
exclude the former.
Nagel (1897) appears to have been the first to note that anemones
(he used Adamsia) soon cease to accept pieces of filter paper soaked in
fish juice, though they will at first grasp and pass this to the mouth
just like other food. A number of workers have confirmed this, using a
variety of species (for references see Thorpe, 1963). There has been
,
M. J. WELLS
for example, may move away from a place in which it was given a
series of electric shocks hours later, during the following night. From
the time scale, with any other type of animal, it would be not unreasonable to suppose that the move was a result of association of area
and shock ; the animals found the region inhospitable and moved on.
Yet because anemones respond so slowly, the possibility remains that
the shocks do no more than cause a switch in long-term patterns of
activity, with results that show up hours later as an apparently adaptive reaction to remembered circumstances (Pantin, 1952). The very
variety of the long term activities of the animals means (as with
echinoderms) that it is diBcult to distinguish whether they are selecting
one of a series of possible action patterns as a result of experience, that
is learning, or simply switching to any of the available alternatives
when the first doesn’t work.
Bearing this in mind and without, at this stage, prejudging the
issue by defining what would constitute evidence of learning in a
coelenterate, it is worth examining the experiments that have been
held to demonstrate learning in the past. These are :
1. Cessation of responses to repeated stimuli that at f i s t cause
withdrawal.
2. Cessation of responses to inedible objects (usually bits of
filter paper soaked in meat juice) repeatedly presented.
3. Persistence and gradual fading of tidal and diurnal
rhythms in anemones brought into constant conditions.
4. Position habits in individuals.
The first type of experiment is clearly a study of habituation.
Aiptasia, for example, normally withdraws when it is suddenly stimulated by a drip falling into the still water of its aquarium. If the drip
occurs repeatedly at intervals of a minute or so (long enough for the
anemone to expand its tentacles again) the response fades after two to
three trials. If the drips are more than about 5 min apart, it does not
(Jennings, 1905). Such habituative phenomena must be general ;
anemones moved from a quiet environment to aquaria with a brisk
circulation do not remain permanently withdrawn. What is not known
is whether one is dealing with sensory accommodation, or changes sited
more deeply in the nervous system. Most definitions of learning would
exclude the former.
Nagel (1897) appears to have been the first to note that anemones
(he used Adamsia) soon cease to accept pieces of filter paper soaked in
fish juice, though they will at first grasp and pass this to the mouth
just like other food. A number of workers have confirmed this, using a
variety of species (for references see Thorpe, 1963). There has been
,