26
M. J. WELLS
slowed subsequent habituation to a moving shadow and vice versa.
Stimulation by sudden decreases in light intensity maintained habituation to moving shadows, but the reverse, with maintenance of the light
response by shadows, was not found (Clark, 1960a).
Stimuli too weak to evoke a response may nevertheless alter the
subsequent behaviour of the animals. I n experiments on the habituation of Nereis pelagica to sudden increases in light intensity, an increase
from 0-08 to 4.7 ft candles had no effect. But sixty such trials nearly
doubled the rate of subsequent habituation to a larger increase from
0.08 to 33.9 ft candles (Fig. 15).
Clark (1960b) in reviewing his own and previous work on habituation
in polychaetes, concludes that, although in general the rate of habituation to simple stimuli is related to their likely biological significance,
learning not to respond to them is too quick and the effects too
persistent for responses to these to form an effective safeguard against
predators. In the sea most worms must be partly habituated to most
simple stimuli most of the time. It is, however, noticeable that the
fading of responses to more complex stimuli (a combination of mechanical shock and shadow, for example) is much slower than habituation
to any of their components, and Clark suggests that nereids a t least
probably recognize the approach of potential predators from comparatively elaborate complexes rather than from simple " sign stimuli."
B . Further experiments with errant polychetes
Evans (1963a) has shown that certain responses of Nereis virens,
N . diversicolor and Perinereis cultrifera (Grube) can be reversed by
training. Introduced into the narrow channel of the apparatus shown
in Fig. 16, these worms invariably crawled rapidly to the far end. They
were successfully taught not to do this by means of small shocks given
through electrodes fixed in the sides of the passage. Of the three
species Nereis virens learns most rapidly. With trials in quick succession 25 worms averaged 21.5 trials to reach a criterion of three refusals
to run down the passage in three consecutive trials (individuals took
from eleven to forty-nine trials). The other two species averaged
47 and 4743 trials, with a similar spread in individual scores. Control
animals were run down the passage without shocks, with the electrodes
switched on between trials; they continued to respond. In some
preliminary experiments, shocks given outside the apparatus did not
appear to alter the tendency to run down the passage afterwards
(Evans, personal communication).
Following up these results Evans (1963b) did experiments with the
same three species in T-mazes, using shocks as punishment for turning
M. J. WELLS
slowed subsequent habituation to a moving shadow and vice versa.
Stimulation by sudden decreases in light intensity maintained habituation to moving shadows, but the reverse, with maintenance of the light
response by shadows, was not found (Clark, 1960a).
Stimuli too weak to evoke a response may nevertheless alter the
subsequent behaviour of the animals. I n experiments on the habituation of Nereis pelagica to sudden increases in light intensity, an increase
from 0-08 to 4.7 ft candles had no effect. But sixty such trials nearly
doubled the rate of subsequent habituation to a larger increase from
0.08 to 33.9 ft candles (Fig. 15).
Clark (1960b) in reviewing his own and previous work on habituation
in polychaetes, concludes that, although in general the rate of habituation to simple stimuli is related to their likely biological significance,
learning not to respond to them is too quick and the effects too
persistent for responses to these to form an effective safeguard against
predators. In the sea most worms must be partly habituated to most
simple stimuli most of the time. It is, however, noticeable that the
fading of responses to more complex stimuli (a combination of mechanical shock and shadow, for example) is much slower than habituation
to any of their components, and Clark suggests that nereids a t least
probably recognize the approach of potential predators from comparatively elaborate complexes rather than from simple " sign stimuli."
B . Further experiments with errant polychetes
Evans (1963a) has shown that certain responses of Nereis virens,
N . diversicolor and Perinereis cultrifera (Grube) can be reversed by
training. Introduced into the narrow channel of the apparatus shown
in Fig. 16, these worms invariably crawled rapidly to the far end. They
were successfully taught not to do this by means of small shocks given
through electrodes fixed in the sides of the passage. Of the three
species Nereis virens learns most rapidly. With trials in quick succession 25 worms averaged 21.5 trials to reach a criterion of three refusals
to run down the passage in three consecutive trials (individuals took
from eleven to forty-nine trials). The other two species averaged
47 and 4743 trials, with a similar spread in individual scores. Control
animals were run down the passage without shocks, with the electrodes
switched on between trials; they continued to respond. In some
preliminary experiments, shocks given outside the apparatus did not
appear to alter the tendency to run down the passage afterwards
(Evans, personal communication).
Following up these results Evans (1963b) did experiments with the
same three species in T-mazes, using shocks as punishment for turning