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I'm looking for an old sci-fi story about large alien creatures that are immobile, building layers of hard shell around them. They use the planets mobile apex creatures (ape-like creatures) for fertilisation, bringing them inside their bodies and pressing them up against a surface that they bite and scratch at, which causes fertilisation, before the Mother creature digests them. I larremember part of the story being this creatures description of the layers she built around herself, using metals and minerals from the surrounding rocks. She also repositioned her stomach and other internal organs for better use, and there was a large canid type creature that managed to gouge it's way through her early shell, only to fall into and cook in her repositioned stomach.
The first part of the story is told by the female, about her beginning and devolment (from memory) and then later a male and female human explorer come to the planet and are captured by these creatures. The male is kept within the mother for some time till he fertilises her and then she intended to digest him, but somehow they end up communicating and she keeps him around for company. He is there long enough to witness the birth of her babies and plays with and interacts with them inside her big spongy cavity where he lives. He tries to make her understand that the other human he was with was female, a mother herself, but the alien creature is greatly disturbed by this concept as the only "mothers" on the planet are the immobiles like herself. That's as far as I got through the story and would like to finish it, if only I could remember what it was. I keep thinking the term "mother" is relevant but can't find anything online about sci-fi stories with that title. Think it was originally something from my dad's collection that I read back in the 80's but he doesn't remember either.

This is "Mother" from Philip Jose Farmer's story collection Strange Relations. The female creature in question is the Mother, shown here depicted in Barlowe's Guide to Extraterrestrials.

The plot, per the review linked above:

The second best story of the collection follows the emotional opera singer Eddie Fetts who has a mother complex and an unhealthy attachment to the nipple-shaped rubber top of his liquor thermos. His mother, an accomplished pathologist, is a constant factor in his life, especially after Eddie’s wife left because they “couldn’t get together”. Eddie and his mother crash land on an unusual planet where Farmer’s skill at describing unusual aliens manifests itself. Eddie and his mother are captured, after being lured by a mating scent, and placed inside different immobile hilltop dwelling aliens.

He soon discovers that these aliens are all female, they impregnate themselves by capturing roving animal life like himself, and they feed their children inside the womb by producing a stew generated by captured animals and water syphoned via long tubes from the ground. Eddie literally returns to the womb and discovers that he strangely likes it there and takes an active part nurturing the young. Of course, the alien mother, Polyphema, gains great prestige having a talking male mobile.