LEARNING BY MARINE INVERTEBRATES
41
The Onchidium experiments do not necessarily invalidate explanations of homing by other species based on topographical learning.
Onchidium is a pulmonate of a rather specialized type and is thus
related only remotely to Siphonaria and hardly at all to the other,
prosobranch, limpets. It is highly probable that the homing habits of
these forms have developed independently. A reinvestigation of the
whole subject in terms of the information potentially available from
orientated features of the environment (polarization of the sky, brightness, slope and humidity gradients) is clearly needed at this stage.
C . Other evidence of learning by molluscs
Chiton, like the gastropods, seems to have some sense of the geography of its immediate surroundings. Crozier (1921) showed that
individuds did not always return “ home ” to the underside of rocks
by their outgoing paths, and considers their performance an “ incipient
stage in a kind of homing instinct.” Bounlangb (1927, cited in Pelseneer,
1935-a source often quoted because it contains names and dates but
no further details of references) apparently did similar experiments.
Among the gastropods, Verlaine (1936) has shown that the performance of the predaceous Natica, which bores holes in the shells of
bivalves, gets better as the animal grows up. Young individuals as
often as not bore their holes in unsuitable places where the shell is
thick or the soft parts withdrawable ; larger Natica make their incision
over the gonad in nearly every case. A similar state of affairs has been
recorded for Nucella lapillus L., which mainly feeds on barnacles.
Fischer-Piette (1936) records a remarkable series of behavioural
changes in a whole population of these whelks, following the overrunning of their barnacle beds by mussels. Apparently the whelks not only
took to feeding on the mussels, but also got more adept a t it as time
went on. Fischer observed that the proportion of shells found with
holes drilled in obviously unsuitable places, such as from the inside of
empty valves, fell progressively during one generation of the whelks,
and remained low throughout the next generation, by now wholly
devoted to mussel-eating. With the subsequent elimination of many
of the mussels, barnacles were able to re-establish and the whelk
population reverted to its original food.
Leaving aside a matter of the inheritance of acquired characters
that would seem to be implied by some of Fischer’s observations, it is
dear that neither this, nor Verlaine’s report is necessarily indicative
of learning. Both are, however, well documented accounts of behavioural
changes that might well form the basis of work on the effect of individual experienoe on feeding.
0 ’
41
The Onchidium experiments do not necessarily invalidate explanations of homing by other species based on topographical learning.
Onchidium is a pulmonate of a rather specialized type and is thus
related only remotely to Siphonaria and hardly at all to the other,
prosobranch, limpets. It is highly probable that the homing habits of
these forms have developed independently. A reinvestigation of the
whole subject in terms of the information potentially available from
orientated features of the environment (polarization of the sky, brightness, slope and humidity gradients) is clearly needed at this stage.
C . Other evidence of learning by molluscs
Chiton, like the gastropods, seems to have some sense of the geography of its immediate surroundings. Crozier (1921) showed that
individuds did not always return “ home ” to the underside of rocks
by their outgoing paths, and considers their performance an “ incipient
stage in a kind of homing instinct.” Bounlangb (1927, cited in Pelseneer,
1935-a source often quoted because it contains names and dates but
no further details of references) apparently did similar experiments.
Among the gastropods, Verlaine (1936) has shown that the performance of the predaceous Natica, which bores holes in the shells of
bivalves, gets better as the animal grows up. Young individuals as
often as not bore their holes in unsuitable places where the shell is
thick or the soft parts withdrawable ; larger Natica make their incision
over the gonad in nearly every case. A similar state of affairs has been
recorded for Nucella lapillus L., which mainly feeds on barnacles.
Fischer-Piette (1936) records a remarkable series of behavioural
changes in a whole population of these whelks, following the overrunning of their barnacle beds by mussels. Apparently the whelks not only
took to feeding on the mussels, but also got more adept a t it as time
went on. Fischer observed that the proportion of shells found with
holes drilled in obviously unsuitable places, such as from the inside of
empty valves, fell progressively during one generation of the whelks,
and remained low throughout the next generation, by now wholly
devoted to mussel-eating. With the subsequent elimination of many
of the mussels, barnacles were able to re-establish and the whelk
population reverted to its original food.
Leaving aside a matter of the inheritance of acquired characters
that would seem to be implied by some of Fischer’s observations, it is
dear that neither this, nor Verlaine’s report is necessarily indicative
of learning. Both are, however, well documented accounts of behavioural
changes that might well form the basis of work on the effect of individual experienoe on feeding.
0 ’