374
M. V. L. BENNETT
and prolonged. However they are still somewhat smaller than in the
main organ of Torpedo. Although the normal discharge of the accessory
organ is unknown, these findings suggest that the discharge involves
repetitive and fused responses of the single cells similar to what is
observed in the rajids (see the next section), It seems likely that the
accessory organ is used either in an active electrosensory system or in
communication, although there is no direct evidence for this suggestion.
The torpedinids like most other elasmobranchs have ampullae of Lorenzini, which are electroreceptors (see Chapter 11, this volume). A
number of other small torpedinids are known, some of which live in
the deep seas and are blind (Lissmann, 1958). Possibly their main
electric organs are used in electrolocation. It is not known whether
these species have accessory organs.
3. RAJIDS
The skates or ordinary rays are a large cosmopolitan group of marine
fish comprising six or more genera and many species. They are weakly
electric, but unlike freshwater species they emit pulses only infrequently.
The discharges are sufficiently inconspicuous that a major taxonomic
work on the group “Fishes of the Western North Atlantic” (Bigelow and
Schroeder, 1953) makes no mention of the fact that these fish are electric.
Other rays (suborder Myliobatoidea ) apparently lack electric organs.
The electric organs of rajids are located in the tail in the center of
the most lateral bundle of longitudinally running muscle fibers (Fig. 1).
The organs are spindle-shaped and run most of the length of the tail.
They are much greater in length than in diameter. The electrocytes are
oriented anteroposteriorly and are innervated on their anterior faces.
Each cell lies in a small connective tissue compartment. Two types of
electrocytes have been described, the cup-shaped and the disc-shaped
(Fig. 11). However, these terms are not particularly descriptive of the
morphological differences. Cup-shaped cells lie at the anterior margin
of their connective tissue chamber. Often they are convex posteriorly,
which accounts for their name. Both faces are relatively smooth at the
light microscopic level of resolution. Electron microscopy reveals a
relatively small number of tubules invaginating into the innervated
face and a somewhat greater number in the uninnervated face (Mathewson et al., 1961). Disc-shaped cells lie nearer the posterior of their
chambers. The posterior, uninnervated faces have a large number of
protuberances tens of microns in diameter and length (Fig. 11). Probably there are more invaginating tubules in these faces than in cupshaped cells. Both classes of cells contain striated filamentous material
M. V. L. BENNETT
and prolonged. However they are still somewhat smaller than in the
main organ of Torpedo. Although the normal discharge of the accessory
organ is unknown, these findings suggest that the discharge involves
repetitive and fused responses of the single cells similar to what is
observed in the rajids (see the next section), It seems likely that the
accessory organ is used either in an active electrosensory system or in
communication, although there is no direct evidence for this suggestion.
The torpedinids like most other elasmobranchs have ampullae of Lorenzini, which are electroreceptors (see Chapter 11, this volume). A
number of other small torpedinids are known, some of which live in
the deep seas and are blind (Lissmann, 1958). Possibly their main
electric organs are used in electrolocation. It is not known whether
these species have accessory organs.
3. RAJIDS
The skates or ordinary rays are a large cosmopolitan group of marine
fish comprising six or more genera and many species. They are weakly
electric, but unlike freshwater species they emit pulses only infrequently.
The discharges are sufficiently inconspicuous that a major taxonomic
work on the group “Fishes of the Western North Atlantic” (Bigelow and
Schroeder, 1953) makes no mention of the fact that these fish are electric.
Other rays (suborder Myliobatoidea ) apparently lack electric organs.
The electric organs of rajids are located in the tail in the center of
the most lateral bundle of longitudinally running muscle fibers (Fig. 1).
The organs are spindle-shaped and run most of the length of the tail.
They are much greater in length than in diameter. The electrocytes are
oriented anteroposteriorly and are innervated on their anterior faces.
Each cell lies in a small connective tissue compartment. Two types of
electrocytes have been described, the cup-shaped and the disc-shaped
(Fig. 11). However, these terms are not particularly descriptive of the
morphological differences. Cup-shaped cells lie at the anterior margin
of their connective tissue chamber. Often they are convex posteriorly,
which accounts for their name. Both faces are relatively smooth at the
light microscopic level of resolution. Electron microscopy reveals a
relatively small number of tubules invaginating into the innervated
face and a somewhat greater number in the uninnervated face (Mathewson et al., 1961). Disc-shaped cells lie nearer the posterior of their
chambers. The posterior, uninnervated faces have a large number of
protuberances tens of microns in diameter and length (Fig. 11). Probably there are more invaginating tubules in these faces than in cupshaped cells. Both classes of cells contain striated filamentous material