Ernst Haeckel (1834-1919) was a German biologist, philosopher, and artist, who published detailed naturalistic drawings of thousands of domestic and previously unknown species. His 1904 volume Kunstformen der Natur contains terrific illustrations of radiolarias, sponges, and jellyfish, among many other species.

Haeckel considered biology closely related to art, and his research shows a particular fascination for the non-random forms of nature: symmetric unicellulars, snail shells, mimicry patterns. His drawings are said to have inspired early artists and architects of the Art Nouveau period. Some of the marine and microbic creatures in his tabloids would even work a great deal as inspiration for sci-fi characters and extraterrestrical life forms!

Quite elaborate wikipedia articles in English and German provide more information about his life, work, and evolutionary theories. This site provides downloadable high resolution scans of all 100 tabloids from Kunstformen der Natur.