Structure and mechanical behaviour of the compass depressors of Paracentrotus lividus (Lamarck) (Echinodermata: Echinoidea)

The compass depressors connect the compass ossicles of the echinoid lantern to the inner edge of the perignathic girdle. They are essentially collagenous ligaments with, on one side, a thin layer of muscle cells. This paper describes briefly the ultrastructure of these components and provides information on their mechanical behaviour, including evidence that the tensility of the collagenous component is under physiological control.