husk, nut shell
殼 centers on a hard outer covering that encloses and protects something inside: a husk or a nut shell.
殼 is the traditional form of 殻. It combines 殳 (a hand holding a tool) with a left side that likely represented a hard outer covering. The character originally depicted striking or removing a husk or shell.
The right side 殳 looks like a hand with a tool, and the left side resembles a hard covering. Imagine using a tool to crack open a nut shell or husk.
For カク, picture a nut shell cracking open with a sharp 'kaku' sound: kaku -> カク, as the husk splits apart.