The Collaborative International Dictionary
Hydrachnid
Hydrachnid \Hy*drach"nid\, n. [Hydr- + arachnid.] (Zo["o]l.) An aquatic mite of the genus Hydrachna. The hydrachnids, while young, are parasitic on fresh-water mussels.
Wiktionary
hydrachnid
n. (context zoology English) An aquatic mite of the genus (taxlink Hydrachna genus).