a1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Lack of understanding of the graptolite nema and virgula is probably the main cause of the general failure to arrive at a general mode of life theory for planktonic species. The nema and virgula are shown to be different structures. The nema may have had a rhabdosome rotational function at early growth stages; the primary function of the virgula may have been rhabdosomal rotation, whilst a secondary function was as a stipe growth guide. Rotational function would be inapplicable for pseudovirgulae and in synrhabdosomes.
(Received October 02 1995)
(Accepted January 11 1996)