Summary. 1. A behavioral response conforming to defining features of the jamming avoidance response (JAR) previously reported in Eigenmannia and Apteronotus of the Cypriniformes is found in Gymnarehus of the Mormyriformes.2. Other parallel specializations of these groups are noted, of which the most relevant is the character of the electric organ discharge (EOD); it is quasisinnsoidal, high in repetition rate and highly regular in each of the genera. The same features are found in Sternopygus but it lacks a JAR.3. The EOD is compared in Gymnarchus, Eigenmannia, Apteronotus and Sternopygus, in respect to power spectrum and regularity.4. Other special features of the EOD in Gymnarchus are described, including "singing"and a miniature EOD of a different frequency from the main EOD.5. The JAR in Gymnarchus, compared to Eigenmannia and Apteronotus is longer in latency, slower in reaching plateau, smaller in maximum frequency shift and best excited by a stimulus frequency closer to its own. The voltage gradient threshold (<2.5 ~tV/cm) is higher and the dynamic range smaller. Some correlations with habit of life are suggested. 6. Two types of electroreceptors seem particularly relevant to the JAR. They are similar to the T and P units already reported in Eigenmannia but substantial differences require separate designations; we call them Type S and Type 0 units. 7. Type S units are like T units but spontaneous at high rates and phase coding over a limited intensity range near threshold. Over a wide range of intensity, including much of the physiological range normally encountered the S unit cannot encode intensity.8. Type 0 units are like P units but usefully coding in a narrow intensity range; they are often unable to reach 1 : 1 following. The threshold is usually about 20 db higher than in S units.9. The filter properties of both types are those of a bandpass filter. Whereas the 0 units are sharply tuned to the EOD frequency, the S units have a fiat passband over a range of about 150 Hz, and sharp cutoffs (about 50 db/octave) on both the high and low frequency sides.

The descending connections from the brainstem to the spinal cord in Eigenmannia sp. were demonstrated using the horseradish peroxidase (HRP) technique. The spinal cord was transected and HRP crystals were deposited in the cut. The point of transection was located at varying distances from the head in different specimens. In all experiments, cells were labeled in both the rhombencephalic and mesencephalic tegmentum. No labeled cells were found in the cerebellum, the lateral-line lobes, the torus semicircularis, the tectum mesencephali, the hypothalamus, the diencephalon or the telencephalon. Labeled neurons were found in the ventrolateral column, nucleus formatio reticularis (NFR) inferior, NFR medius, NFR superior pars superior and pars suprema, NFR tegmenta mesencephali lateralis, nucleus vestibularis magnocellularis and nucleus fasciculi longitudinalis medialis. Furthermore, the Mauthner cells and the neurons of the pacemaker nucleus were filled with HRP granules. The neurons labeled were predominantly the large ones of more than 25 µm in diameter which are very conspicuous along the brainstem. The number of these neurons in the different nuclei varied from animal to animal, however, the number of labeled neurons increased monotonically at a similar rate in all brainstem nuclei with more rostrally located transection sites. In a second series, the number of neurons terminating in a small number of segments independent of absolute position along the body axis was assessed using two different fluorescent dyes. Within tolerable statistical limits, this number was found to be constant, corroborating the data obtained with HRP. A possible interpretation of the data is placed in the context of physiological data previously presented.

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