Final answer:
Cumaceans are the unique group of hermaphroditic crustaceans capable of discharging both eggs and sperm through a common duct.
Step-by-step explanation:
The small group of hermaphroditic crustaceans that is unique in discharging both eggs and sperm through a common duct is the cumaceans. Unlike most crustaceans, which are usually dioecious, cumaceans possess the rare capability to release both types of gametes via the same pathway. This is an exception in the crustacean world, as crustaceans typical reproductive modes include separate sex individuals such as isopods and amphipods, or sequential hermaphroditism as seen in some other species.