What's the difference between acalephae and acalephan?

Acalephae

Definition:

• (n. pl.) A group of Coelenterata, including the Medusae or jellyfishes, and hydroids; -- so called from the stinging power they possess. Sometimes called sea nettles.

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Acalephan

Definition:

• (n.) One of the Acalephae.

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