tectibranchiata

from The Century Dictionary.

• A division of gastropods, usually held as an order or a suborder of Gastropoda, which have a single lateral gill, covered by the mantle (whence the name), and whose shell, varying in size according to the genus, is very small and sometimes concealed.

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

• noun plural (Zoöl.) An order, or suborder, of gastropod Mollusca in which the gills are usually situated on one side of the back, and protected by a fold of the mantle. When there is a shell, it is usually thin and delicate and often rudimentary. The aplysias and the bubble shells are examples.