Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 2
नारद उवाचपुरुषं पुरञ्जनं विद्याद्यद् व्यनक्त्यात्मन: पुरम् ।एकद्वित्रिचतुष्पादं बहुपादमपादकम् ॥ २ ॥
nārada uvācapuruṣaṁ purañjanaṁ vidyādyad vyanakty ātmanaḥ purameka-dvi-tri-catuṣ-pādaṁbahu-pādam apādakam
The great sage Nārada Muni continued: You must understand that Purañjana, the living entity, transmigrates according to his own work into different types of bodies, which may be one-legged, two-legged, three-legged, four-legged, many-legged or simply legless. Transmigrating into these various types of bodies, the living entity, as the so-called enjoyer, is known as Purañjana.
How the spirit soul transmigrates from one type of body to another is nicely described here. The word eka-pāda, “one-legged,” refers to ghosts, for it is said that ghosts walk on one leg. The word dvi-pāda, meaning “biped,” refers to human beings. When he is old and invalid, the human being is supposed to be a triped, or three-legged, because he walks with the help of a stick or some kind of cane. Of course, the word catuṣ-pāda refers to quadrupeds, or animals. The word bahu-pāda refers to those creatures who have more than four legs. There are many insects, such as the centipede, and also many aquatic animals that have many legs. The word apādaka, meaning “without legs,” refers to serpents. The name Purañjana indicates one who enjoys possessing different types of bodies. His mentality for enjoyment in the material world is accommodated by different types of bodies.