Interspecific variation in the chelonian primary visual system.

The primary visual system of 21 species of turtles, distributed among nine of the existing 12 families, were studied by autoradiography. In all species, contralateral visual projections exist to 15 targets: two hypothalamic structures (nucleus suprachiasmaticus and n. periventricularis), three major thalamic visual centres (nucleus ovalis, n. geniculatus lateralis ventralis and n. geniculatus lateralis dorsalis) and two minor thalamic targets (nucleus dorsolateralis anterior and n. ventrolateralis), five pretectal sites (nucleus geniculatus pretectalis, n. opticus pretectalis ventrolateralis, n. lentiformis mesencephali, n. posterodorsalis and n. griseus tectalis), two strata of the optic tectum (stratum opticum and s. fibrosum et griseum superficiale), and a single tegmental target (nucleus opticus tegmenti). In contrast to the stability of contralateral visual projections, their ipsilateral counterparts varied considerably between species, being limited to the hypothalamus in some species, and involving the majority of the primary visual centres in others. This variation is not readily explainable in terms of taxonomic position or of differences in mode of life.