Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a French-made love story (Leonard Maltin calls it a "wickedly amusing comedy) about a professional dominatrix (or mistress - "la maitresse" in French) Ariane (Bulle Ogier) who falls in love with a man, Olivier (Gerard Depardieu) who she met when he was breaking into her apartment to rob her. Ariane has a normal apartment on one floor, but when you slide a coffee table forward, a ladder appears that takes you downstairs to her dungeon. It has all sorts of bondage and torture devices and tools, including a dentist's chair and a crucifix. The scenes of Ariane working with her clients are sometimes very graphic and difficult to watch, but there's a scene of a horse dying in a slaughterhouse that is even harder to watch.
As if all of Ariane's dominatrix costumes and masks and tools and torture chambers are not weird enough for the audience (who, in 1975, had probably never seen anything like them) somebody decided she needed to have some eccentric hobbies. We that she keeps carniverous plants that she feeds live ants, and she has an illuminated aquarium on the front side of her black bathtub that is filled with goldfish and snakes. Olivier asks if the snakes eat the fish and she says "sometimes." He asks if they can get into the bathtub and she says they can't. An aquarium is not a suitable habitat for most snakes. They would probably die from the unhealthy conditions, so I wonder if Ariane is torturing the snakes as she does her clients. Although it is made clear in the movie that her clients have signed forms requesting particular treatment and she is only allowed to do what they ask. It's "torture" by consent. In fact, Olivier implies that the clients are actually the ones in charge because they tell her what to do. She replies that she uses her imagination to improve on what they tell her to do. I wish the set designers had used their imaginations better than designing a bathtub aquarium full of snakes.
The film was probably made in France so the snakes are most likely a European species, maybe Grass Snakes, it's hard to see them well enough to tell what they are.