Final answer:
In crustaceans, the photoreceptor cells that make up the compound eye are called ommatidia, which are microscopically small units each functioning like a tiny eye.
Step-by-step explanation:
The photoreceptor cells in the crustacean compound eye are called ommatidia. Each ommatidium serves as a microscopic eye, part of the overall compound eye structure found in crustaceans. Whereas photoreceptors in vertebrates involve pigments like rhodopsin, found in the rod and cone cells, crustaceans have a different arrangement. The compound eyes of crustaceans are made up of many tiny units called ommatidia, each with its own cornea, lens, and photoreceptor cells that capture light and contribute to the animal's ability to see.