The development of the muscular system of the gastropod mollusc Patella has been thoroughly studied. As a result, two larval retractors, the main and accessory larval retractor, had been described in the larva of Patella. These muscles were supposed to be responsible for the retraction of the larval body into the shell. Previously no larval extensors, which would be responsible for the extension of the larval body out of the shell, have been described. Using cell-lineage tracer injection and phalloidin staining of muscles, a newly-discovered muscle is herein identified in the larva of Patella coerulea. A functional model is presented in which two muscles are responsible for the extension of the larva, the newly-discovered muscle together with the previously recognized but inaccurately named accessory larval retractor. In our model, the latter muscle does not seem to function as a retractor but rather as an extensor.