Amphioxus

From Èlia Benito-Gutiérrez http://www.biolsci.org/v02p0149.htm (Rearranged in her words):

Since the evolution of the nervous system in the Animal Kingdom is currently under major debate in Evolutionary Developmental Biology and Comparative Embryology, the elaboration of extremely complex nervous systems is a major success of evolution; however, little data have helped yet to unravel how a nervous system develops and evolves to complexity thus making the amphioxus a key position to tackle this exciting issue. Whatever its actual position, as the sister group of vertebrates, as the earliest chordate, or as (less likely) the earliest deuterostome, its privileged phylogenetic position sets amphioxus in the appropriate place to cast light on some of the main transitions that occurred during animal evolution: the origin of deuterostomes, the origin of Chordates, or the origin of vertebrates, all of them correlated with novel evolutionary features, and some of them linked to the elaboration of the most complex nervous systems.

"Amphioxi are seldom more than 8 cm (3 inches) long and resemble small, slender fishes without eyes or definite heads." http://www.britannica.com/EBchecked/topic/21580/amphioxus