CAMBRIAN PROTOCHORDATA, THEIR ORIGIN AND SIGNIFICANCE

A survey of the available palaeontological and recent evidence points to the following interim conclusions: (a) animals belonging to the cho rdate lineage occurred in the Middle Cambrian, when they were already separated in at least three branches; (b) in the Middle Cambrian fossi ls the notochord may have been not or not entirely differentiated, nor did they have a perfect bilateral symmetry: myomeres, and most probab ly the nervous system bring ''off symmetry'' by half a segment between the right and left half of the body, just as they presently are in th e Acrania and, incompletely, in the Copelata and Cyclostomata; (c) in one of the Middle Cambrian taxa the skull was represented by skeletal material corresponding to the nonsegmental portions of that of living vertebrates and, as in living cyclostomes, probably derived entirely f rom neuroectodermal cells from the placodes and neural crest; (d) chor dates may have evolved from Vendian animals showing the same type of i ncomplete bilateral symmetry; (e) the possible affinities of the Cambr ian and recent taxa are briefly discussed.