364
M. V. L. BENNElT
protruding when they are practically invisible except for the two small
black spots of their pupils. From this position they gulp down unwary
minnows passing overhead. The electric organs lie just behind the eyes
( Fig. 1 ) , and they are in point of fact modified from extraocular muscles
(Dahlgren, 1914, 1927). Other members of the same family, Uranoscopidae, are very similar in appearance but lack electric organs.
The electrocytes are large flattened cells that lie in the horizontal
plane and are densely innervated on their dorsal surface by branches of
the very large oculomotor nerves (Fig. 5 ) . The ventral surface has many
short processes or papilli that markedly increase its surface. This surface
is further increased by many small invaginating tubules or canaliculi
(Mathewson et al., 1961; Wachtel, 1964). About 150-200 layers of cells
are arranged in series, one above the other (Dahlgren and Silvester,
1906). Each layer of cells in parallel consists of about four large cells
(approximately 5 mm in diameter in a 20-cm fish) surrounded by about
10 smaller ones. A single large cell can also spiral around to overlap
itself to some extent.
The organ discharges are pulses making the dorsal surface negative
(and presumably also making the inside of the mouth positive, although
this has not been directly verified). Responses to handling the fish range
from single pulses up to trains of several tens of pulses at frequencies of
about 5&100/sec. Trains of pulses are also emitted when the fish is capturing small minnows (Pickens and McFarland, 1964). The pulses are
about 5 msec in duration. The amplitude is somewhat variable, particularly at the beginning of a train but is up to about 5 V recorded from a
fish 20-30 inches in length with its dorsal surface in air (Fig. 6 ) . The
voltage across the organ is reduced somewhat if the discharge is evoked
while the fish is immersed in seawater. The pulses are generated synchronously by the two organs.
The effectiveness of the discharge in aiding the capture of prey has
not been demonstrated, but the unstimulated fish emits pulses rarely, if
ever, at other times. Large fish can reach 40 cm in length, and the discharge is sufficiently strong that it is easily detected when the fish are
handled (Dahlgren and Silvester, 1906). Even a small fish can cause
mild discomfort if one's hands are quite wet and have a number of
minor cuts (personal observation).
Only the innervated (dorsal) face of the electrocytes is active during
organ discharges (Bennett and Grundfest, 1961b). The responses are
PSPs and are monophasic depolarizations of about the Same shape and
duration as the organ pulses. The innervated membrane does not respond
at all to depolarization but behaves linearly; it is electrically inexcitable.
The response that is evoked by stimuli applied directly to the organ is
M. V. L. BENNElT
protruding when they are practically invisible except for the two small
black spots of their pupils. From this position they gulp down unwary
minnows passing overhead. The electric organs lie just behind the eyes
( Fig. 1 ) , and they are in point of fact modified from extraocular muscles
(Dahlgren, 1914, 1927). Other members of the same family, Uranoscopidae, are very similar in appearance but lack electric organs.
The electrocytes are large flattened cells that lie in the horizontal
plane and are densely innervated on their dorsal surface by branches of
the very large oculomotor nerves (Fig. 5 ) . The ventral surface has many
short processes or papilli that markedly increase its surface. This surface
is further increased by many small invaginating tubules or canaliculi
(Mathewson et al., 1961; Wachtel, 1964). About 150-200 layers of cells
are arranged in series, one above the other (Dahlgren and Silvester,
1906). Each layer of cells in parallel consists of about four large cells
(approximately 5 mm in diameter in a 20-cm fish) surrounded by about
10 smaller ones. A single large cell can also spiral around to overlap
itself to some extent.
The organ discharges are pulses making the dorsal surface negative
(and presumably also making the inside of the mouth positive, although
this has not been directly verified). Responses to handling the fish range
from single pulses up to trains of several tens of pulses at frequencies of
about 5&100/sec. Trains of pulses are also emitted when the fish is capturing small minnows (Pickens and McFarland, 1964). The pulses are
about 5 msec in duration. The amplitude is somewhat variable, particularly at the beginning of a train but is up to about 5 V recorded from a
fish 20-30 inches in length with its dorsal surface in air (Fig. 6 ) . The
voltage across the organ is reduced somewhat if the discharge is evoked
while the fish is immersed in seawater. The pulses are generated synchronously by the two organs.
The effectiveness of the discharge in aiding the capture of prey has
not been demonstrated, but the unstimulated fish emits pulses rarely, if
ever, at other times. Large fish can reach 40 cm in length, and the discharge is sufficiently strong that it is easily detected when the fish are
handled (Dahlgren and Silvester, 1906). Even a small fish can cause
mild discomfort if one's hands are quite wet and have a number of
minor cuts (personal observation).
Only the innervated (dorsal) face of the electrocytes is active during
organ discharges (Bennett and Grundfest, 1961b). The responses are
PSPs and are monophasic depolarizations of about the Same shape and
duration as the organ pulses. The innervated membrane does not respond
at all to depolarization but behaves linearly; it is electrically inexcitable.
The response that is evoked by stimuli applied directly to the organ is