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The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

The weakly electric fish Gnathonemus petersii uses its electric sense to actively probe the environment. Its highly mobile chin appendage, the Schnauzenorgan, is rich in electroreceptors. Physical measurements have demonstrated the importance of the position of the Schnauzenorgan in funneling the fish's self-generated electric field. The present study focuses on the trigeminal motor pathway that controls Schnauzenorgan movement and on its trigeminal sensory innervation and central representation. The nerves entering the Schnauzenorgan are very large and contain both motor and sensory trigeminal components as well as an electrosensory pathway. With the use of neurotracer techniques, labeled Schnauzenorgan motoneurons were found throughout the ventral main body of the trigeminal motor nucleus but not among the population of larger motoneurons in its rostrodorsal region. The Schnauzenorgan receives no motor or sensory innervation from the facial nerve. There are many anastomoses between the peripheral electrosensory and trigeminal nerves, but these senses remain separate in the sensory ganglia and in their first central relays. Schnauzenorgan trigeminal primary afferent projections extend throughout the descending trigeminal sensory nuclei, and a few fibers enter the facial lobe. Although no labeled neurons could be identified in the brain as the trigeminal mesencephalic root, some Schnauzenorgan trigeminal afferents terminated in the trigeminal motor nucleus, suggesting a monosynaptic, possibly proprioceptive, pathway. In this first step toward understanding multimodal central representation of the Schnauzenorgan, no direct interconnections were found between the trigeminal sensory and electromotor command system, or the electrosensory and trigeminal motor command. The pathways linking perception to action remain to be studied.

Gnathonemus petersii use electrolocation to navigate in unfamiliar environments. The goal of these experiments was to determine whether fish could learn the location of a fixed aperture after interference with selected sensory input. By manipulating environmental cues (aperture height and water depth) and comparing the fish's performance, the contributions of the electrosensory system, vision, and hydrostatic pressure were examined. The fish's task was to find a circular aperture in a wall dividing a 200-litre aquarium into two equal compartments. In experiment 1, the position of the aperture was raised by 10.1 cm after the fish had become familiar with its original location. In experiment 2, the water level was raised by 10 cm (leaving the aperture unchanged). When the aperture was raised, intact fish found the new aperture with no difficulty, whereas blind, electrically 'silent', and sham-operated fish were slow finding the new position. When the water level was raised, all fish increased the height at which they contacted the wall, increased their electric-organ discharge (EOD) rate, and located the aperture. This increase, in response to the rapid change in water depth, suggests that all fish used hydrostatic pressure cues to maintain depth orientation, and that those fish that learned the aperture height had used hydrostatic cues to locate its position. The data suggest that G. petersii develop an internal representation based on an electrosensory central expectation and hydrostatic cues. The fish develop a sensory 'image' of their immediate environment and associate a specific image with a specific depth. As the environment becomes more familiar, the fish apparently attend less to electrosensory information and navigate according to the internal representation, relying primarily on hydrostatic pressure cues.

We investigated the electrolocation performance of the weakly electric fish, Gnathonemus petersii, in novel and familiar environments. By selectively interfering with the fish's sensory input, we determined the sensory channels necessary for navigation and orientation. The fish's task was to locate a circular aperture (diameter: 64 mm) in a wall dividing a 200–1 aquarium into two equal compartments. To assess the fish's performance, we measured (1) the time it took the fish to locate the aperture, (2) the height at which it contacted the divider, (3) its electric organ discharge rate, and (4) the frequency of divider crossings. In the first experiment (novel environment), 50 naive G. petersii assigned to five groups of 10 fish each (intact, blind, electrically “silent,” blind and “silent,” and shamoperated animals) were tested with the aperture presented randomly in one of three positions (aperture center: 7.6, 17.7, 27.8 cm from the bottom). In a novel environment, G. petersii depend on active electrolocation. Despite the changing aperture position, over the 15 trials, fish with a functioning electric organ found the aperture, whereas those without one did not. The electric organ discharge rate was inversely correlated with the amount of time spent searching for the aperture. In a second experiment (familiar environment) 20 intact fish learned the position of a fixed aperture. When we subsequently denervated the electric organ in 10 of these animals, their performance did not differ significantly from that of their conspecifics. Thus, once the fish were familiar with the aperture's position, they no longer depended on active electrolocation. We interpret and discuss this behavior as evidence for a “central expectation” and discuss its possible role in electronavigation.

In four species of mormyrid weakly electric fish (Pollimyrus isidori, Gnathonemus petersii, Mormyrus rume, and M. caschive) the electric signalling and motor behaviour during the solution of a conditioned electrolocation task was investigated. All fish had to discriminate between two objects of different electric properties which were identical otherwise. The behaviour of the fish before, during, and after electrolocation of the objects was recorded and analysed. The same types of stereotyped motor behaviours (‘probing motor acts’, PMAs) were observed in all fish during probing of the objects. Independent of the occurrence of PMAs, ‘probing’ was accompanied by a regular pattern (regularization) of electric organ discharges (EODs). Interval durations between successive discharges were kept constant on an individual specific level which was maintained in successive tests under the same conditions. This level was negatively correlated with the spectral frequency of peak power of a single EOD but was also influenced by additional parameters such as motivation. It did not depend, however, on the type of electrolocation target. Before and after investigation of the objects, the discharge rhythm was variable with inter-pulse intervals spanning a wider range than during ‘probing’. In contrast to discharge rhythm, there was no change in waveform or spectral composition of single EODs of a fish, regardless of behavioural context. In conclusion I suggest that regularization of electric signalling is an important factor during electrolocation in all mormyrids but its level is not critical. No special behavioural strategies exist which might explain different electrolocation abilities of the four mormyrid species.

Investigations on the diurnal rhythm of the motoric and electric activity were performed with the electric fish Gnathonemus petersii. A group of 11 conspecifics as well as single fishes showed an increased electric and motoric activity at night, especially at the beginning and end of the dark phase and also during feeding. These normal diurnal rhythms were changed for at least 3 days by the aggressive behaviour of two fishes who were set together for the first time. During this time an order of precedence was established.

The present study has explored the optomotor response in two species of mormyrid fish, Gnathonemus petersii and Brienomyrus niger. Both species tended to follow a black and white striped stimulus pattern under illumination levels of 6, 12, and 60 lx. The optomotor response ceased to occur under 540 lx. The behavioral data support earlier histological findings implicating the mormyrid retina in dim light vision.

Several species of Mormyrid weakly electric fish have a mobile chin protuberance that serves as a mobile antenna during prey detection, tracking behaviors, and foraging for food. It has been proposed that it constitutes a fovea of the electrosensory system. The distribution of the three types of receptor organs involved in active imaging of the local surroundings, prey detection, and passive electroreception, and their central projection to the electrosensory lobe (ELL), have been studied in Gnathonemus petersii. Density distributions were compared for different body regions. Primary afferent projections were labeled with biocytin or biotinylated dextrans. This showed that there is considerable central "over-representation" of the mandibular and nasal regions of the sensory surface involved in electrolocation, at the expense of the other body regions investigated. This over-representation is not a mere effect of the very high density of receptor organs in these areas, but is found to be due to central magnification. This magnification differs between the subclasses of electroreceptors, suggesting a functional segregation in the brain. We conclude that the chin protuberance and the nasal region are the regions of greatest sensitivity for the resistive, capacitive, and low-frequency characteristics of the environment, and are probably most important in prey detection, whereas other regions of the skin with a lesser resolution and sensitivity to phase distortion of the EOD, in particular the trunk, are probably designed for imaging larger, inanimate features of the environment. Our data support the hypothesis that the chin appendage and nasal region are functionally distinct electrosensory foveae.

Gnathonemus petersii and other African weakly electric fish (Mormyridae) produce electric signals in order to sense their environment, a process called active electrolocation. During active electrolocation electrical images of objects in the environment are cast onto a mosaic of electroreceptors, which are embedded in the animal's skin. In Gn. petersii and four other species of mormyrid fish, the densities of electroreceptor organs (mormyromasts and ampullary organs) were determined at various skin regions, including the chin, the nasal region and the back. In all species investigated, the highest mormyromast densities were found at the chin, followed by the nasal region. In addition to this comparative approach, we investigated the electroreceptive epidermis of Gn. petersii in more detail. At the moveable chin appendix, the Schnauzenorgan, scanning electron microscopy analysis helped to elucidate the mosaic-like distribution and the morphology of the electroreceptor organs and their embedding in the epidermis. Because the number of mormyromasts increased very little when fish got bigger, receptor organ densities decreased with standard length. Mormyromast density at the Schnauzenorgan, especially at its tip, highly exceeded that at all other skin regions. Along the Schnauzenorgan, receptor densities decreased exponentially from the tip towards the base. At the nasal region, mormyromast densities were more than three times higher compared to the rest of the fish's head and trunk. A similar distribution to that of the mormyromasts was found for the ampullary electroreceptor organs, which are used for passive electroreception. Our findings support the hypothesis that at least in Gn. petersii the Schnauzenorgan and the nasal region are electroreceptive foveae.

The cerebellum of mormyrid fish is of interest for its large size and unusual histology. The mormyrid cerebellum, as in all ray-finned fishes, has three subdivisions--valvula, corpus, and caudal lobe. The structures of the mormyrid valvula and corpus have been examined previously, but the structure of the mormyrid caudal lobe has not been studied. The mormyrid caudal lobe includes a posterior caudal lobe associated with the electrosense and an anterior caudal lobe associated with lateral line and eighth nerve senses. In this article we describe cellular elements of the posterior caudal lobe and of the eminentia granularis posterior (EGp) in the mormyrid fish Gnathonemus petersii. The EGp gives rise to the parallel fibers of the posterior caudal lobe. We used intracellular injection of biocytin, extracellular injection of biotinylated dextran amine, and immunohistochemistry with antibodies to gamma-aminobutyric acid, inositol triphosphate receptor I, calretinin, and Zebrin II. The histological structure of the posterior caudal lobe is markedly irregular in comparison to that of the corpus and the valvula, and a tight modular organization of cerebellar elements is less apparent here. Most Purkinje cell bodies are in the middle of the molecular region. Their dendrites are only roughly oriented in the sagittal plane, extend both ventrally and dorsally, and branch irregularly. Climbing fibers terminate only on smooth dendrites near the soma. Most Purkinje cell axons terminate locally on eurydendroid cells that project outside the cortex. The results provide an additional variant to the already large set of different cerebellar and cerebellum-like structures.

An examination of the permanent bony structures of the anal fin complex in the mormyrid fish, Gnathonemus petersii, revealed two new structural sexual dimorphisms: longer proximal pterygiophores and wider anal fin rays in males than in females. Both structures are thought to facilitate the male’s courtship-associated anal fin reflex. Adult male mormyrid fishes are characterized by a dorsally directed indentation of the posterior body wall (anal fin indentation). The expression of this indentation in males, presumably driven by anal fin musculature, was correlated with the fish’s gonadal state: large indentations were associated with high gonado-somatic indices and small indentations with low indices.

In pairs of interacting Gnathonemus petersii, a mormyrid electric fish, dominant individuals were larger (longer body). Pairs of interacting fish of similar body size emitted more parallel displays at the onset of an interaction. Over the course of 10 min interactions, head butting increased and parallel display decreased. This decrease occurred primarily in pairs that contained a prior resident and intruder, as compared with two intruders. The patterns of electric organ discharge during parallel display, and the possible sensory modalities mediating this behaviour are discussed.

The mormyrids are well known for their remarkable electrogenic and electrolocation capabilities and exceptionally large cerebellum that may account for much of their total oxygen consumption. Mormyrids living in oxygen-deficient waters may use oxygen efficiently, protecting the brain from hypoxia damage; and/or brain size may be reduced. This study compares the TFL, gill lamellar density, gill lamellar area, total gill surface area, and brain size of two species of mormyrids from extremely hypoxic waters in Uganda (Gnathonemus victoriae and Petrocephalus catostoma) to two open-water species (Mormyrus kannume and Gnathonemus longibarbis) from the same region. In addition, interdemic variation was considered by comparing swamp populations of G. victoriae and P. catostoma to open-water populations of the same species. Total gill surface area of all species fell within the upper range for freshwater fishes. However, there were both intraspecific and interspecific differences in gill characters. Interdemic comparisons showed larger gill size in swamp-dwelling populations. Brain size varied among species; larger brains were characteristic of species from well-oxygenated waters. Large gill surface area may permit survival of mormyrids in oxygen-stressed environments; however, mechanisms compensating for hypoxia seem to be inadequate to support a brain size as large as that seen in fish from well-oxygenated waters.

Male Gnathonemus petersii (Mormyridae) exhibit two structural, sexually dimorphic characters: anal-fin ray bone expansion and an indentation of the posterior ventral body wall (formerly described as anal-fin indentation). Females lack this bone expansion, but may show a slight indentation. Morphometric data on both characters were obtained from radiographs of 414 fish (males and females) ranging in size from 60 to 276 mm Ls. Both body wall indentation and bone expansion began to develop in males of about 120 mm Ls. At 160–180 mm, the sex ratio of fish with expansion to fish without expansion was 50 : 50. Androgens seem to affect the expression of both these sexual dimorphisms. Bone expansion may provide increased bone surface for muscle attachment and thus facilitate the anal-fin reflex during courtship behaviour.

The retina of the weakly electric fish Gnathonemus petersii is a so-called grouped retina where photoreceptors are bundled. These bundles are regarded as functional units and this type of retinal specialization is uniquely found in teleosts. To understand how this anatomical organization influences visual information processing we investigated the morphology and distribution of retinal ganglion cells (GCs) and the response properties of retinal afferents terminating in the major retinorecipient area, the optic tectum. GCs were classified based on their dendritic morphology (dendritic field diameters 280 μm: giant GCs). Within these classes subtypes were distinguished based on the ramification patterns of the dendrites in the sublaminae of the inner plexiform layer. Properties of presumed optic nerve terminals were investigated in the optic tectum using extracellular recordings. Physiological classes could be observed based on their response to visual stimuli (on; off; on-off, and fast units). Receptive field sizes and spatiotemporal properties were classified and the topographical representation of the visual space was mapped in the tectum. Gratings of low spatial frequencies were best responded to and followed up to high temporal frequencies (>30 Hz). Most of the recorded units were directionally selective. No evidence of distorted topographies in the tectum was found, i.e., no overrepresentation of the retina was seen in the tectum opticum. The grouped retina of G. petersii seems to be optimized for the detection of large, fast objects in an environment of low optical quality.

Most of the information about the anatomy of the fish's cranial nerves was collected in the first two decades of this century. Experimental analysis of the VIIth, IXth, and Xth cranial nerves by modern tract tracing techniques started about 20 years ago. Several species have been investigated to date, including one species of Agnatha (Myxinoidea), two species of elasmo-branchs, and species of some orders of Teleostei like Cyprinidae, Siluriformes, Perciformes, and Gadidae. The Sensory and motor nuclei of the VIIth, IXth, and Xth cranial nerves of Gnathonemus petersii were studied by anterograde and retrograde axoplasmatic transport of horseradish peroxidase and cobaltous lysine complex. The sensory nuclei form a continuous column of cells in the brain stem extending caudal to the obex. The rostral one-fourth of this column is occupied by the overlapping terminals of the VIIth and IXth nerves. The vagus nerve has 5 roots. The first 4 of these innervate the gills and the fifth supplies viscera. Afferents from the gills terminate ipsilaterally rostral to the obex in topographic order and their terminal fields overlap. Viscerosensory fibers terminate ipsilaterally in the obex region and bilaterally in the commissural nucleus of Cajal. The facial motor nucleus is located rostral to the sensory nucleus. Facial motoneurons have pear-shaped and multipolar perikarya. Their axons form a rostrally directed knee before leaving the brain. The motoneurons of the IXth and Xth nerves have a common cell column. The vagal motoneurons form a periventricular, a medial, and an intermediate cell group rostral to the obex. In the obex region and also caudal to it, a lateral and a caudal group can be distinguished. Vagal motoneurons show a topographic arrangement that is similar to that of the sensory vagal projections. The majority of motoneurons have pear-shaped perikarya and ventrolaterally oriented dendrites. In the caudal nucleus the dendrites extend dorsally and overlap the terminals of sensory fibers. The axons form a dorsolaterally directed arch before joining the sensory roots. Since G. petersii uses its electrosensory system primarily for detection of food, its gustatory system is less developed than in other fishes, which possess a large number of taste buds. © 1992 Wiley-Liss, Inc.

Immunocytochemistry has demonstrated unexpected heterogeneity among cerebellar Purkinje cells. For example, monoclonal antibody Mab anti-zebrin II reveals parasagittal bands of immunoreactive Purkinje cells in the mammalian cerebellum, but reveals a non-sagittal cerebellar compartmentation pattern in goldfish and gymnotiform fish. The present paper investigates the cerebellar compartmentation pattern, as reflected in the zebrin II distribution, in two other teleosts, the electric mormyrid fish Gnathonemus petersii with its large and regularly built gigantocerebellum, and the electrosensory osteoglossomorph teleost Xenomystis nigri, by using light as well as electron microscopic immunohistochemical techniques. Zebrin II is expressed only in Purkinje cells, where it is present in the cytoplasm of all neuronal compartments, including spines, distal and proximal dendrites, the cell body, and the initial part, as well as terminal boutons of the axon. Other types of cerebellar neurons, including the eurydendroid projection neurons, are zebrin II-negative. In Gnathonemus, zebrin II-positive Purkinje cells are present in the large caudolateral part of the valvula, in lobes C2, C3, and C4 of the corpus, and in the anterior as well as the posterior part of the caudal cerebellar lobe. Zebrin II-negative Purkinje cells are present in a continuous region encompassing the rostromedial part of the valvula, the lobus transitorius, lobe C1 and the ventral part of lobe C2, and in a small, lateral zone of the posterior part of the caudal lobe. In Xenomystis, all Purkinje cells, including those in the medial valvula and the posterior part of the caudal lobe, appear to react with mab anti-zebrin II. This more widespread distribution may be due to the presence of a second antigenic polypeptide in this species. On the basis of the present findings, it is concluded that the mormyrid lobus transitorius, lobe C1, and the ventral part of lobe C2 probably belong to the valvula, while the corpus is restricted to the dorsal part of lobe C2, lobe C3, and lobe C4. The functional significance of zebrin II expression for different subsets of teleostean Purkinje cells remains unclear, since comparisons of different teleosts reveal no general correlation with particular afferent or efferent connections, nor with special morphological features such as a dendritic palisade pattern or different arrangements of the Purkinje cell bodies. A comparison between mammals and teleosts suggests that a distinct parasagittal cerebellar zonation in teleosts is absent, and the major part of the teleostean cerebellum may be considered as a single (midsagittal) cerebellar zone, with about the same width as one mammalian parasagittal zone.

Mormyrids are one of two groups of teleost fishes known to have evolved electroreception, and the concomitant neuroanatomical changes have confounded the interpretation of many of their brain areas in a comparative context, e.g., the diencephalon, where different sensory systems are processed and relayed. Recently, cerebellar and retinal connections of the diencephalon in mormyrids were reported. The present study reports on the telencephalic and tectal connections, specifically in Gnathonemus petersii, as these data are critical for an accurate interpretation of diencephalic nuclei in teleosts. Injections of horseradish peroxidase into the telencephalon retrogradely labeled neurons ipsilaterally in various thalamic, preglomerular, and tuberal nuclei, the nucleus of the locus coeruleus (also contralaterally), the superior raphe, and portions of the nucleus lateralis valvulae. Telencephalic injections anterogradely labeled the dorsal preglomerular and the dorsal tegmental nuclei bilaterally. Injections into the optic tectum retrogradely labeled neurons bilaterally in the central zone of area dorsalis telencephali and ipsilaterally in the torus longitudinalis, various thalamic, pretectal, and tegmental nuclei, some nuclei in the torus semicircularis, the nucleus of the locus coeruleus, the nucleus isthmi and the superior reticular formation, basal cells in the ipsilateral valvula cerebelli, and eurydendroid cells in the contralateral lobe C4 of the corpus cerebelli. Weaker contralateral projections were also observed to arise from the ventromedial thalamus and various pretectal and tegmental nuclei, and from the locus coeruleus and superior reticular formation. Tectal injections anterogradely labeled various pretectal nuclei bilaterally, as well as ipsilaterally the dorsal preglomerular and dorsal posterior thalamic nuclei, some nuclei in the torus semicircularis, the dorsal tegmental nucleus, nucleus isthmi, and, again bilaterally, the superior reticular formation. A comparison of retinal, cerebellar, tectal, and telencephalic connections in Gnathonemus with those in nonelectrosensory teleosts reveals several points: (1) the visual area of the diencephalon is highly reduced in Gnathonemus, (2) the interconnections between the preglomerular area and telencephalon in Gnathonemus are unusually well developed compared to those in other teleosts, and (3) two of the three corpopetal diencephalic nuclei are homologues of the central and dorsal periventricular pretectum in other teleosts. The third is a subdivision of the preglomerular area, rather than an accessory optic or pretectal nucleus, and is related to electroreception. The preglomerulo-cerebellar connections in Gnathonemus are therefore interpreted as uniquely derived characters for mormyrids.

The optic nerve and the retinal projections were studied in a mormyrid fish, Gnathonemus petersii, by using Fink-Heimer, HRP, cobalt labeling, and autoradiographic tracing techniques. The retinal fibers terminate bilaterally in the following places: suprachiasmatic nucleus, dorsolateral optic nucleus, optic nucleus of the posterior commissure, cortical nucleus, ventral pretectal area, optic tectum, and the accessory optic terminal field. The number of uncrossed fibers is relatively high in the suprachiasmatic nucleus, but negligibly small in the other retinal terminal fields. In the lateral geniculate nucleus and pretectal nucleus only crossed retinal fibers could be detected. The visual system of Gnathonemus is compared to that of other fishes, amphibians, and reptiles and the possible homologies are proposed. The comparison points to the conclusion that the visual system is less developed in Gnathonemus. This nocturnal species lives in turbid waters and has a special electric sense which may permit compensation for the reduced visual capacity.

The projection regions of electroreceptor and mechanical lateral line afferents in electric fish of the mormyridae family are described. Electroreceptor afferents from the posterior dorsal skin run in the dorsal branch of the posterior lateral line nerve. Electroreceptor afferents from ventral skin and mechanical lateral line afferents and efferents run in the ventral branch of the nerve. Horseradish peroxidase (HRP) injections into each branch resulted in filling of its central terminals with the marker enzyme. The method yields a Golgi-like staining of afferent terminals, allowing some aspects of their morphology to be described. Comparison of results from dorsal and ventral branch injections shows the separate medullary regions to which electroreceptor and mechanical afferents project, and also demonstrates four separate somatotopic maps within the electroreceptor region. Mechanical afferents end predominantly ipsilaterally in nucleus anterior and eminentia granularis as has been suggested by others. Ipsilateral endings in nucleus octavius are also seen. Electroreceptor afferents end exclusively in the cortex and nucleus of posterior lateral line lobe (PLLL). Within the cortex there are three distinct maps of the skin surface which are separated from each other by discontinuities in the cellular layers. Somatotopic mapping is also present in the nucleus of PLLL though it is less precise than in the cortical zones. Large club endings of the cells of this nucleus are filled with HRP. Labeled cells are seen within a small midline nucleus located at the level of the eighth nerve just above the medial longitudinal fasciculus. These are probably the cell bodies of lateral line efferents.

Scanning electron microscopic studies were conducted on the sensory epithelia of the auditory portions of the ears in teleost species representing wide taxonomic diversity. A number of the features of the ears investigated resem- bled features found in other teleost species, although some major exceptions to earlier patterns were found, particularly in the saccular sensory epithelium. The saccular maculae of all but one species contained basically similar ciliary bundles on the sensory hair cells while there was some significant variation on the lagenar maculae. Hair cell orientation patterns on the sacculus contained four orientation groups in all of the species, other than the mormyrid, Gnathonemus, which only had two groups. Lagenar maculae had two orientation groups, and the orientation patterns were similar to one another. The most divergent form of lagenar macula was found in Gnathonemus. These data, combined with data from earlier investigations, provide a broad overview of the surface features of the ear in teleost fishes. Most significantly, it now appears that there are at least five different saccular hair cell orientation patterns among teleost fishes, and all of these patterns are found spread through many major teleost taxa. While there is some similarity in ear structures among some groups of closely related species, such as the Elopomorpha and the Gadiformes, it is becoming more apparent that there is extensive convergence in a number of features of the teleost ear that most likely reflect similar selective pressures during the evolution of the ear. The nature of these selective pressures, however, are not well understood.

The central distribution of the posterior lateral line nerve (NLLp) was studied in the weakly electric African fish, Gnathonemus petersii. Lesions were made in the ganglion of NLLp and the degeneration traced by means of the Fink-Heimer technique. NLLp distributes to the ipsilateral posterior lateral line lobe, anterior lateral line lobe, and lobus caudalis cerebelli. The posterior lateral line lobe is laminated and NLLp terminates in an organized manner within three of its six laminae. In addition the projection of NLLp to the posterior lateral line lobe is topographic. The projection of NLLp to the anterior lateral line lobe is more diffusely organized.