M. V. L. BENNETT
456
scopus to branchial muscles in torpedinids, to pectoral muscles in
Mahpterurus, to axial and tail muscles in the remainder. In electrocytes
of Astroscopus, rajids, and mormyrids striations are easily seen under
the light microscope. The orientation however is quite unrelated to
the cell axis. Electron microscopy shows well-organized Z lines and thin
filaments in both Astroscopus and rajids. Gymnurchus electrocytes have
similar but less regular structures (Schwartz et al., 1971). In the
mormyrid Gnethonemus there are both thick and thin filaments and
Z lines. The filaments are not very well aligned, however, and the cross
bridges of normal muscle are certainly reduced in number if not absent
altogether. Sarcoplasmic reticulum, associated in normal muscle with
control of tension, is lacking in most regions or very sparse. It is not
surprising then that the cells do not move, although it would be
interesting to know the biochemical “deficiency,” if any, in the contractile
machinery.
Electrocytes of the other three evolutionary lines of electric fish,
torpedinids, gymnotids, and Mahpterurus, all have fine but disorganized
filaments without Z lines (Wachtel, 1964; Schwartz et al., 1971). The
origin of these tissues from muscle is less obvious in the adult, although
the development of the organ from myoblast-like tissue has been
demonstrated histologically (Johnels, 1956; Keynes, 1961; see also
Fritsch, 18%). The physiological and pharmacological properties
strongly support derivation from muscle. Some histological aspects of
electric organ development have also been described for Gymnurchus,
Astroscopus, and Mormyrus (Dahlgren, 1914, 1927; Szabo, 1961~).
An interesting problem is the ontogeny of excitability in electrocytes
of marine fish. Rajid electrocytes develop from rather normal appearing
muscle fibers (Ewart, 1892). If they are normal, the spike generating
membrane present in tail muscle of the adult (Grundfest and Bennett,
1961) would have to be lost. Embryological material is readily obtainable
for this group unlike that for most other electric fish. Increase in the
number of electrocytes during growth may also provide an opportunity
for experimental analysis. Gymnotids regenerate their tails including
electric organs (Ellis, 1913) and electric eels apparently add layers of
electrocytes as they increase in length (Keynes, 1961).
The existence of a neurogenic accessory organ in the chin region of
Adontosternarchus (Section 11, D, 1, g ) has developmental as well as
evolutionary implications. It suggests a kind of preadaptation but one not
involving a purposeful change toward a structure that has survival
advantage only at some later time. A probable early stage in the chin
organ development was maintained oscillatory activity of a group of
electroreceptors (see Chapter 11, this volume) that served as a weakly
electric organ for other receptors. The afferent fibers would then have
456
scopus to branchial muscles in torpedinids, to pectoral muscles in
Mahpterurus, to axial and tail muscles in the remainder. In electrocytes
of Astroscopus, rajids, and mormyrids striations are easily seen under
the light microscope. The orientation however is quite unrelated to
the cell axis. Electron microscopy shows well-organized Z lines and thin
filaments in both Astroscopus and rajids. Gymnurchus electrocytes have
similar but less regular structures (Schwartz et al., 1971). In the
mormyrid Gnethonemus there are both thick and thin filaments and
Z lines. The filaments are not very well aligned, however, and the cross
bridges of normal muscle are certainly reduced in number if not absent
altogether. Sarcoplasmic reticulum, associated in normal muscle with
control of tension, is lacking in most regions or very sparse. It is not
surprising then that the cells do not move, although it would be
interesting to know the biochemical “deficiency,” if any, in the contractile
machinery.
Electrocytes of the other three evolutionary lines of electric fish,
torpedinids, gymnotids, and Mahpterurus, all have fine but disorganized
filaments without Z lines (Wachtel, 1964; Schwartz et al., 1971). The
origin of these tissues from muscle is less obvious in the adult, although
the development of the organ from myoblast-like tissue has been
demonstrated histologically (Johnels, 1956; Keynes, 1961; see also
Fritsch, 18%). The physiological and pharmacological properties
strongly support derivation from muscle. Some histological aspects of
electric organ development have also been described for Gymnurchus,
Astroscopus, and Mormyrus (Dahlgren, 1914, 1927; Szabo, 1961~).
An interesting problem is the ontogeny of excitability in electrocytes
of marine fish. Rajid electrocytes develop from rather normal appearing
muscle fibers (Ewart, 1892). If they are normal, the spike generating
membrane present in tail muscle of the adult (Grundfest and Bennett,
1961) would have to be lost. Embryological material is readily obtainable
for this group unlike that for most other electric fish. Increase in the
number of electrocytes during growth may also provide an opportunity
for experimental analysis. Gymnotids regenerate their tails including
electric organs (Ellis, 1913) and electric eels apparently add layers of
electrocytes as they increase in length (Keynes, 1961).
The existence of a neurogenic accessory organ in the chin region of
Adontosternarchus (Section 11, D, 1, g ) has developmental as well as
evolutionary implications. It suggests a kind of preadaptation but one not
involving a purposeful change toward a structure that has survival
advantage only at some later time. A probable early stage in the chin
organ development was maintained oscillatory activity of a group of
electroreceptors (see Chapter 11, this volume) that served as a weakly
electric organ for other receptors. The afferent fibers would then have