These are from Die acraspeden Medusen der deutschen Tiefsee-Expedition: 1898-1899, the first of two volumes on jellyfish written by Ernst Vanhöffen, a jellyfish scholar who was thrilled to be on the Valdivia Expedition.
As the Biodiversity Heritage Library describes it:
The expedition covered over 32,000 nautical miles, visited 268 stations around the West Coast of South Africa, the Gulf of Guinea, the Antarctic Sea, and a large portion of the Indian Ocean, and collected a wealth of specimens (many of which were new to science) from depths greater than 13,000 feet. Over seventy scientists worked for forty years to publish twenty-four volumes detailing the expedition’s scientific discoveries.
The flowing, graceful subjects here are actually by drawn by Ew. H. Rübsaamen and lithographed by Fritz Winter, the shipboard artists.
As a pulp horror fan, as well as a science fan, I have a growing hunch that in the 1920s, when HP Lovecraft was describing tentacled cosmic horrors, he was thinking less about the squid and octopus and more about the medusans – both the microscopic creatures and elegant jellyfish like these. The geometry seems totally logical, mathematically inevitable, and not very human. This also would have been fairly popular science in the first couple of decades of the 20th century.
Visually, it’s all very hypnotic, isn’t it?