The telencephalon of the sea catfish Galeichthys felis.

The sea catfish is a relatively abundant teleost fish. Its placement on the phylogenetic scale remains in question, although its brain resembles grossly those of some of the other teleosts which also possess pedunculated olfactory bulbs. The forebrain of Galeichthys felis consists of two hemispheres which lack lateral ventricles. They are joined at the midline by a thin membrane ventromedially and by the hippocampal and anterior commissures. The telencephalic hemispheres are overlaid by a single, ventrolaterally attached membrane which is continuous rostrally with the roofs of the olfactory ventricles. Six basic nuclear regions are evident in the telencephalon of the sea catfish: a dorsomedial, a ventromedial, a ventral, a lateral, a dorsal and a central. Dorsomedially, the primordial hippocampal formation is divided into an anterior continuation, a primordial dentate gyrus, a primordial cornu ammonis and a primordial subiculum. Ventromedially, the precommissural septum consists of the medial septal nucleus (pars dorsalis and pars ventralis), the lateral septal nucleus and the nuclei of the hippocampal and anterior commissure. Ventrally, the medial and the lateral zones and the medial island constitute the tuberculum olfactorium. The primordial general pallium comprises the dorsal area. Laterally, the primordial piriform cortex and the prepiriform region overlie the relatively large primordial amygdaloid complex, which includes an anterior anygdaloid nucleus, a primordial corticomedial amygdaloid nucleus and a primordial basolateral amygdaloid nucleus. The hyperstriatum, the neostriatum, and paleostriatum augmentatum and the paleostriatum primitivum constitute the central region. Basic fiber systems in Galeichthys felis which are homologous to those of higher vertebrates include the medial and the lateral olfactory tracts, the fornix, the medial and the lateral forebrain bundles, the stria medullaris pathways and the stria terminalis. In spite of the lack of lateral telencephalic ventricles in this form, nuclear areas were defined which, on the basis of topography, cellular morphology and fiber pathways, were homologized with the six basic regions found in the forebrain of higher vertebrates. The basic pattern of the fiber pathways present in the sea catfish corresponds to that found in the other submammalian vertebrates.