Answer:
It is called pen and it is located at the dorsal side
Step-by-step explanation:
Squid belongs to the class Cephalopoda, subclass Coleoidea which is characterized by internal shell (unlike subclass Nautiloidea in which shell is located externally).
The shell of the squid (the pen) in very reduced and small, formed from chitin and located at the dorsal side. The pen provides attachment for muscles and it gives the squid stiffness.
Most likely, this internal shell evolved from the external shell, because it is believed the ancestral of the organisms from the Coleoidea was probably nautiloid-like.