42
M. J. WELTAS
Bivalves in general do not seek their food. It comes to them. They
have a rather complete and simple means of responding to enemies.
Not surprisingly, the only aspect of learning that has been reported in
lamellibranchs is habituation. The animals respond to simple stimuli,
such as passing shadows (Patten, 1886; Hecht, 1924; Kennedy, 1963)
and generally cease to do so when these are frequently repeated. Where
the matter has been specifically investigated this change in behaviour
would seem attributable to sensory adaptation. Thus Hecht (1919,
1924) working with N y a aremria L. showed that the mobilization of
photosensitive substances in dark adaptation takes a matter of hourB,
and argues that it is more reasonable to attribute the failure of the
siphon-withdrawal response to exhaustion of photosensitive substances
in the receptors rather than to “ learning ”. In the absence of experiments with a variety of stimuli, or localized stimuli-studies comparable
with those done on annelids-there would seem no reason to dispute this
view.
,
VI. PLATYHELMINTHES
The current outbreak of work on flatworm learning has so far been
based entirely on freshwater triclads, notably Dugesia ( = Plumria).
A recent review of this subject is included in Jacobson (1963). Information about marine flatworms is limited to Hovey’s (1929) study of
learning in Leptoplana, a polyclad. Though by no means recent,
Hovey’s work is included here for comparison with learning by other
marine animals, and because it is in some ways more convincing than
much of the Dugesia work.
Hovey trained Leptoplum to remain still when a light was switched
on, thus reversing a normal response of the animal, which is negatively
phototactic. After 12 hin the dark, theanimals (n=17) were exposed to
a bright light (300 W at 25 cm!) for 5 min. Every time a worm moved it
was touched on the snout, and the number of touches counted. After
a 5-min training period the light was switched out for 30 min before a
further training session, followed by another half hour in the dark and
so on. The results of this experiment are summarized in Fig. 23 ; the
number of touches needed to keep the worms immobile fell progressively, though they never ceased to respond altogether. Explanations based on sensory adaptation, motor fatigue or physical damage to
the snout were eliminated by control groups subjected to light alone,
or trained after surgical removal of the snout. The control group of
twelve worms used to test the effect of light alone was subsequently
trained in the same manner as the rest. They learned more slowly
than the other experimental animals, having to be touched on average
M. J. WELTAS
Bivalves in general do not seek their food. It comes to them. They
have a rather complete and simple means of responding to enemies.
Not surprisingly, the only aspect of learning that has been reported in
lamellibranchs is habituation. The animals respond to simple stimuli,
such as passing shadows (Patten, 1886; Hecht, 1924; Kennedy, 1963)
and generally cease to do so when these are frequently repeated. Where
the matter has been specifically investigated this change in behaviour
would seem attributable to sensory adaptation. Thus Hecht (1919,
1924) working with N y a aremria L. showed that the mobilization of
photosensitive substances in dark adaptation takes a matter of hourB,
and argues that it is more reasonable to attribute the failure of the
siphon-withdrawal response to exhaustion of photosensitive substances
in the receptors rather than to “ learning ”. In the absence of experiments with a variety of stimuli, or localized stimuli-studies comparable
with those done on annelids-there would seem no reason to dispute this
view.
,
VI. PLATYHELMINTHES
The current outbreak of work on flatworm learning has so far been
based entirely on freshwater triclads, notably Dugesia ( = Plumria).
A recent review of this subject is included in Jacobson (1963). Information about marine flatworms is limited to Hovey’s (1929) study of
learning in Leptoplana, a polyclad. Though by no means recent,
Hovey’s work is included here for comparison with learning by other
marine animals, and because it is in some ways more convincing than
much of the Dugesia work.
Hovey trained Leptoplum to remain still when a light was switched
on, thus reversing a normal response of the animal, which is negatively
phototactic. After 12 hin the dark, theanimals (n=17) were exposed to
a bright light (300 W at 25 cm!) for 5 min. Every time a worm moved it
was touched on the snout, and the number of touches counted. After
a 5-min training period the light was switched out for 30 min before a
further training session, followed by another half hour in the dark and
so on. The results of this experiment are summarized in Fig. 23 ; the
number of touches needed to keep the worms immobile fell progressively, though they never ceased to respond altogether. Explanations based on sensory adaptation, motor fatigue or physical damage to
the snout were eliminated by control groups subjected to light alone,
or trained after surgical removal of the snout. The control group of
twelve worms used to test the effect of light alone was subsequently
trained in the same manner as the rest. They learned more slowly
than the other experimental animals, having to be touched on average