Shell
Bhagavad-gita As It Is
BG Chapters 1 - 6
One who is able to withdraw his senses from sense objects, as the tortoise draws its limbs within the shell, is firmly fixed in perfect consciousness.
Srimad-Bhagavatam
SB Canto 1
The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.
Once both the atheists and the theists were engaged in producing nectar from the sea so that all of them could become deathless by drinking it. At that time the Mandarācala Hill was used as the churning rod, and the shell of Lord Tortoise, the incarnation of Godhead, became the resting place (pivot) of the hill in the seawater.
SB Canto 2
The gigantic universal form of the Personality of Godhead, within the body of the universal shell, which is covered by sevenfold material elements, is the subject for the virāṭ conception.
SB Canto 3
This universe, or the universal sky which we can visualize with its innumerable planets, is shaped just like an egg. As an egg is covered by a shell, the universe is also covered by various layers. The first layer is water, the next is fire, then air, then sky, and the ultimate holding crust is pradhāna. Within this egglike universe is the universal form of the Lord as the virāṭ-puruṣa. All the different planetary situations are parts of His body.