The central nervous system of the brachiopterygian fish Erpetoichthys calabaricus

In this paper a survey is presented of the macrostructure and microstructure of the central nervous system of the brachiopterygian fish Erpetoichthys calabaricus, based partly on personal observations and partly on the literature. The spinal cord resembles in its structural organization that of other fish groups. It is, however, remarkable that at lower levels the ventral and dorsal horns are represented by isolated islands of gray without a zona intermedia. In the rhombencephalon the gray matter is arranged in four longitudinal zones or areas which have been termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. These longitudinal areas coincide largely, but not entirely, with the so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus o the solitary tract, several somatosensory centres. The cerebellum comprises, apart from a pair of large auriculae, a well developed central body. The latter consists of bilateral symmetrical halves which have invaginated into the ventricular cavity. The rostral part of this central body represents the valvula cerebelli, whereas its caudal part represents the corpus cerebelli. Microscopically, the cerebellum contains a molecular layer, a zone of Purkinje cells and a zone of granule cells, but these structures are not arranged in the usual laminated pattern. The medium-sized mesencephalon comprises a ventral tegmentum and a dorsal tectum. The former can be subdivided into a medial zone and a lateral zone. The medial zone may be considered as a direct rostral continuation of the, mainly somatomotor, rhombencephalic area ventralis. The lateral tegmental zone contains, apart from a rather small torus semicircularis, a huge, submeningeally situated, cell mass, which is known as the torus lateralis. The functional significance of this centre is entirely unknown. The tectum shows a distinct laminar pattern which in some respects is more reminiscent to that of anurans than to that of actinopterygian fishes. The fibre systems in the brain stem and spinal cord have not been studied with experimental techniques so far. However, the following pathways could be clearly distinguished in our normal material: (1) several bundles of primary afferent fibres in the rhombencephalic alar plate, (2) a distinct lateral lemniscus, (3) a large fasciculu longitudinalis medialis, comprising vestibulospinal, vestibulomesencephalic and reticulospinal fibres, as well as the giant fibre of Mauthner, (4) a tectobulbar fibre system, (5) a fasciculus retroflexus, (6) a number of fasciculi tegmentales +