1. Coghill, G. E. J. comp. Neurol., 19, 83–105 (1909).
2. Breder, C. M. Zoologica, New York, 4, 159–297 (1926).
3. Gray, J. J. exp. Biol., 10, 88–104 (1933).
4. Gray, J. Q. Jl microsc. Sci., 94, 551–578 (1953).
5. Gray, J., in Essays in Marine Biology (edit. by Marshall, S. M., and Orr, A. P.), 1–16 (Oliver and Boyd, Edinburgh, 1953).