Image courtesy of Thomson JA, Simpson JJ (1909), An Account of the Alcyonarians Collected by the Royal Indian Marine Survey Ship Investigator in the Indian Ocean. 1–316 and Plates I-IX. Download larger version (jpg, 477 KB).
A figure from a taxonomic monograph published in 1909 shows hand drawings of sclerite variation among 15 species of octocorals from the Indian Ocean. Identification of most octocoral species comes down to this level of microscopic detail.