1. Morgan, T. H., Zool. Bull., 1, 287 (1898); Anat. Anz., 17, 1 (1900).
2. Needham, A. E., Proc. Zool. Soc. Lond., 123, 111 (1953).
3. Tufail, M., Ph. D. thesis, University of Exeter (1960).
4. Vorontzova, M. A., and Liosner, L. D., A sexual Propagation and Regeneration (Pergamon Press, Oxford, 1960).
5. Needham, A. E., J. Exp. Biol, 24, 220 (1947).