562
M. V. L. BENNETT
of the ineffectiveness of electric organs in offensive roles it seems likely
that the defensive functions were more important in initial stages of
conversion from purely electrosensory to defensiveoffensive organs. In
agreement with Lissmann’s hypothesis the eel retains a weakly electric
organ and has many weakly electric relatives. Some torpedinids also have
a weakly electric organ, and some of the little known benthic forms are
blind and may use their organs in an active electrosensory system. No
evidence for weakly electric organs has been found in catfish, and the
intermediate stages leading to the electric catfish are presumably lost.
Evidence for active or passive electroreception is lacking for the stargazers.
Lissmann further suggested, although less explicitly, that electroreceptors and electric organs developed concurrently in the evolution
of the weakly electric fish. As lateral line receptors became modified to
detect electric signals, muscles became modified to generate electric
signals. In light of subsequent knowledge it seems more reasonable to
believe that passive electrosensory systems were evolved initially and
that electric organs were added subsequently. Most elasmobranches have
ampullae of Lorenzini indicating that the primitive forms giving rise to
rajids and torpedinids had passive electrosensory systems. Many catfish
have electrosensory systems so that the hypothesized weakly electric
ancestor of the electric catfish probably originally had a passive electrosensory system. All known mormyrids and gymnotids have electric organs,
but the presence of the tonic electroreceptors is consistent with a prior
stage possessing a passive electrosensory system. It does seem possible
that the phasic receptors in these groups developed concurrently with the
electric organs and they are certainly adapted to each other in different
fish (Section IV, D ) . The similarities between receptors of mormyrids
and Gymnarchus suggest that Gymnurchus already had an electric organ
when it diverged from the mormyrid group. No data concerning electroreceptors are available from the stargazers, nor is the cerebellum at all
enlarged unlike known electrosensory fish (Section V, C). This group
remains a possible anomaly.
In the initial stages of evolution of passive electrosensory systems,
lateral line receptors presumably lost mechanical sensitivity and developed electrical sensitivity. This transformation probably required little
more than an alteration of the outer face of the receptor cell. It would be
expected that the inner face would already have developed a high electrical sensitivity for detection of sinall microphonic potentials generated
by the outer face (see Section VII). The arrangement of the receptor
cells in an epithelium is similar in all lateral line receptors and their
homologs, and the major change leading to tonic electroreceptors may
M. V. L. BENNETT
of the ineffectiveness of electric organs in offensive roles it seems likely
that the defensive functions were more important in initial stages of
conversion from purely electrosensory to defensiveoffensive organs. In
agreement with Lissmann’s hypothesis the eel retains a weakly electric
organ and has many weakly electric relatives. Some torpedinids also have
a weakly electric organ, and some of the little known benthic forms are
blind and may use their organs in an active electrosensory system. No
evidence for weakly electric organs has been found in catfish, and the
intermediate stages leading to the electric catfish are presumably lost.
Evidence for active or passive electroreception is lacking for the stargazers.
Lissmann further suggested, although less explicitly, that electroreceptors and electric organs developed concurrently in the evolution
of the weakly electric fish. As lateral line receptors became modified to
detect electric signals, muscles became modified to generate electric
signals. In light of subsequent knowledge it seems more reasonable to
believe that passive electrosensory systems were evolved initially and
that electric organs were added subsequently. Most elasmobranches have
ampullae of Lorenzini indicating that the primitive forms giving rise to
rajids and torpedinids had passive electrosensory systems. Many catfish
have electrosensory systems so that the hypothesized weakly electric
ancestor of the electric catfish probably originally had a passive electrosensory system. All known mormyrids and gymnotids have electric organs,
but the presence of the tonic electroreceptors is consistent with a prior
stage possessing a passive electrosensory system. It does seem possible
that the phasic receptors in these groups developed concurrently with the
electric organs and they are certainly adapted to each other in different
fish (Section IV, D ) . The similarities between receptors of mormyrids
and Gymnarchus suggest that Gymnurchus already had an electric organ
when it diverged from the mormyrid group. No data concerning electroreceptors are available from the stargazers, nor is the cerebellum at all
enlarged unlike known electrosensory fish (Section V, C). This group
remains a possible anomaly.
In the initial stages of evolution of passive electrosensory systems,
lateral line receptors presumably lost mechanical sensitivity and developed electrical sensitivity. This transformation probably required little
more than an alteration of the outer face of the receptor cell. It would be
expected that the inner face would already have developed a high electrical sensitivity for detection of sinall microphonic potentials generated
by the outer face (see Section VII). The arrangement of the receptor
cells in an epithelium is similar in all lateral line receptors and their
homologs, and the major change leading to tonic electroreceptors may