Arnold Lang (18 June 1855 – 30 November 1914) was a Swiss naturalist, a comparative anatomist and student of German biologist Ernst Haeckel.

[1] In the autumn of 1891, a European fellowship for the best graduate in class enabled American experimental biologist Lilian Vaughan Morgan to go to Europe and study muscles in chitons at the University of Zurich with Arnold Lang.

Norwegian biologist Kristine Bonnevie studied under Arnold Lang in Zürich in the years 1898-99.

By way of a suggestion from Ernst Haeckel, he published a translation of Jean Baptiste Lamarck's Philosophie zoologique into German (1876).

From his research at Naples, he published in 1884 a massive monograph on Polycladida (marine flatworms), and from 1888 to 1894, he issued a textbook on the comparative anatomy of invertebrates that was subsequently translated into French and English.