monothalamous

from The Century Dictionary.

• Having but one chamber, as some Foraminifera of the family Lituolidæ.
• In botany, single-chambered; having but one compartment; unilocular: applied to galls upon plants, and also rarely (as by Tuckerman) to the apothecia of certain lichens.
• In entomology, having but one cavity: applied to the nests or galls of insects when they have only a single chamber.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Zoöl.) One-chambered.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective zoology Having a single chamber.