Final answer:
The nauplius is the oldest ancestral and most widely-occurring larva in Crustacea, evident in the primitive stages of development of various aquatic crustaceans.
Step-by-step explanation:
The oldest ancestral and most widely-occurring larva in the Crustacea is the nauplius. In the early development stages of aquatic crustaceans, larval stages such as nauplius or zoea are observed.
While a cypris larva is significant in the development of barnacles, the nauplius stage is deemed the most primitive and commonly found across different crustacean species. Crustaceans, which are part of the Phylum Arthropoda, include a variety of species, most of which are aquatic, such as shrimp, lobsters, crabs, and crayfish.