Upon returning to the university of Kristiania, Arnesen began teaching classes and private pupils, but spent her free time studying zoology under Johan Hjort.

This is where Arnesen completed her doctoral studies on the blood vessel system structure in leeches under the supervision of anatomist Arnold Lang.

Here, under Max Carl Wilhelm Weber, she was responsible for a collection of animals that she had grown interested in during her time working on the coasts of Norway: sponges.

She was a member of the executive committee of the Norwegian Women's Union and frequently wrote for newspapers on topics ranging from professional matters to social issues.

Arnesen wrote a text book to be used for high school level zoology, which focused heavily on Charles Darwin's ideas about evolution.