The Collaborative International Dictionary
Mesotheca
Mesotheca \Mes`o*the"ca\, n. [NL., fr. Gr. me`sos middle + ? box.] (Zo["o]l.) The middle layer of the gonophore in the Hydrozoa.
Wiktionary
mesotheca
n. (context zoology English) The middle layer of the gonophore in the Hydrozoa.