The most fundamental of Hindu deities, is the Trinity of Brahma, Vishnu and Shiva - popular deities include Ganesha, Krishna, Hanuman and goddesses like Lakshmi, Durga,Saraswati.
Between the linga and Nandi, is the image of a turtle. Some say that this turtle is the great Akupara, on whose back rests the entire cosmos. Others say, this turtle is Vishnu's Kurma avatar, who held aloft mount Mandara, which the gods used as a churning staff when they churned the cosmic ocean..More importantly, the ancient yogis were impressed by the turtle's ability to shrink its limbs and neck into its shell and escape from the temptations and threats offered by the outside world. It became the symbol of pratyahara, a yogic practice by which mystics dissociated themselves from all sensory stimulation and regressed into the world within.