Spatangoidea, [529], [549] f., [556], [559], [561], [577]

Spatangus, [553], [555];

S. purpureus, [555];

S. raschi, [555]

Specialisation in Metazoa and Volvox compared, [129] f.

Specific gravity of living protoplasm, [13] n.

Spencer, Herbert, on limit of growth, [23]

Spencer, W. B., on Hydroids, [271] n., [279]

Spencerian, fission at limit of growth, [23];

rhythm, [30] f.