The brain of the crossopterygian fish Latimeria chalumnae: A survey of its gross structure

Summary

The macroscopic anatomy of the brain of the single surviving crossopterygian species Latimeria chalumnae is described and depicted. The brain of this fish is slender and elongated. The rhombencephalon is well developed; its ventricular aspect shows four longitudinally arranged ridges which roughly correspond to the functional zones of Herrick and Johnston. The cerebellum comprises two extremely large auriculae and an unpaired, evaginated corpus cerebelli. The mesencephalon is small and does not show any marked differentiation of its surface. In the diencephalon, ventricular sulci mark the boundaries between the epithalamus, dorsal thalamus, ventral thalamus and hypothalamus. The dorsal thalamus protrudes into the ventricular cavity. The telencephalon can be clearly divided into a dorsal pallium and a ventral subpallium. The pallium is represented by a thickened, solid body. It is partly covered by a membranous roof, which in the median plane constitutes an ependymal septum. The subpallium is thin-walled and clearly evaginated. This structure and the ventral part of the pallium enclose a distinct lateral ventricle. The olfactory bulbs are connected with the telencephalon proper by extremely long olfactory peduncles.

Interestingly, the brain of Latimeria appears to have gross structural features in common with all major groups of fish, i.e. the Chondrichthyes or cartilaginous fishes, the Dipnoi or lung fishes and the Actinopterygii or ray-finned fishes. Thus, with respect to the shape of the rhombencephalon and of the vestibulolateral lobe of the cerebellum, Latimeria approaches the chondrichthyan condition; the mesencephalon, the diencephalon and the subpallial parts of the telencephalon share a number of features with their dipnoan homologues, whereas the corpus cerebelli, the pallium and the membranous parts of the telencephalon clearly resemble the corresponding structures in the actinopterygians. No special structural affinities to the amphibians were noticed.