The Brains of Vertebrates

Abstract

The origin of the craniate-vertebrate brain lies in the dark. A phylogenetic reconstruction is of little help, because the echinoderms as putatively most ancient form within the deuterostomes do not possess a centralized, but a penta-partite radial nervous system plus nerve rings which are unlike anything else in the animal kingdom and faintly resembling that of jellyfish. The nervous system of the sister group of echinoderms, the hemichordates, has more resemblance to primitive nervous systems of protostomes than to that of craniates. Among the sister groups of craniates, the urochordates have an extremely simple nervous system, which may be secondarily simplified in the context of their sessile lifestyle. Only the nervous system of the cephalochordata Branchiostoma reveals similarities with the craniates CNS and---most importantly---appears to possess most genes responsible for the ontogenetic development of the craniate brain, as described in the previous chapter.