Final answer:
The apex is the oldest part of the gastropod's shell, formed during the embryonic stage. This point is the center around which the rest of the shell is coiled as the gastropod grows. The shells, made of calcium carbonate, provide a rich fossil record displaying various shapes and exhibit torsion in gastropod development but not related to shell coiling.
Step-by-step explanation:
The oldest part of the shell in gastropods like snails and slugs is called the apex, which is the initial part of the shell formed when the gastropod is an embryo. As the gastropod grows, its shell grows in a coiled manner, forming newer parts of the shell in a spiral around the apex. The shell construction progresses with the older parts remaining near the apex and the newer shell material being added around the opening where the gastropod's body extends to interact with the environment.
Gastropod shells are primarily made of calcium carbonate, and this construction leaves behind a rich fossil record. Noticeable in fossil records are the variations in shell shapes, exhibiting both planospiral and conispiral formations among different species. Moreover, gastropods undergo a unique developmental process called torsion, affecting the arrangement of their bodily organs but this torsion doesn't refer to the coiling of the shell itself.