socket

(Subclass Branchiura):

On third pair of thoracic appendages (thoracopods) of male, socket-like depression in protopod. Receives peg of fourth thoracopod in a peg-and-socket arrangement and serves to hold both thoracopods together during copulation [Stachowitsch, 1992].

(Class Ostracoda):

Hinge structure: pit-like (typically anterior or posterior) depression in hinge margin. Receives tooth of opposite valve. Major depression at end of hinge is termed cardinal socket; those with subdivided floors are termed compound sockets [Stachowitsch, 1992].